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Should We Re-think Regulations and Standards for Lighting at Workplaces? A Practice Review on Existing Lighting Recommendations

Authors:

Abstract

Nowadays lighting projects often include temporal variations of the light, both spectrally and in terms of intensity to consider non-visual effects of light on people. However, as of today there are no specific regulations. Compliance with common lighting standards that address visual aspects of light, often means that only little non-visually effective light reaches the eye. In this practice review we confront existing regulations and standards on visual lighting aspects with new recommendations on non-visual aspects and highlight conflicts among them. We conclude with lighting recommendations that address both aspects.
PERSPECTIVE
published: 13 May 2021
doi: 10.3389/fpsyt.2021.652161
Frontiers in Psychiatry | www.frontiersin.org 1May 2021 | Volume 12 | Article 652161
Edited by:
Shadab Rahman,
Harvard Medical School,
United States
Reviewed by:
Ljiljana Udovicic,
Federal Institute for Occupational
Safety and Health, Germany
Cosmin Ticleanu,
Building Research Establishment,
United Kingdom
Kevin William Houser,
Oregon State University,
United States
*Correspondence:
Oliver Stefani
oliver.stefani@unibas.ch
Specialty section:
This article was submitted to
Sleep Disorders,
a section of the journal
Frontiers in Psychiatry
Received: 11 January 2021
Accepted: 14 April 2021
Published: 13 May 2021
Citation:
Stefani O and Cajochen C (2021)
Should We Re-think Regulations and
Standards for Lighting at Workplaces?
A Practice Review on Existing Lighting
Recommendations.
Front. Psychiatry 12:652161.
doi: 10.3389/fpsyt.2021.652161
Should We Re-think Regulations and
Standards for Lighting at
Workplaces? A Practice Review on
Existing Lighting Recommendations
Oliver Stefani 1,2
*and Christian Cajochen 1,2
1Centre for Chronobiology, Psychiatric Hospital of the University of Basel, Basel, Switzerland, 2Transfaculty Research
Platform Molecular and Cognitive Neurosciences (MCN), University of Basel, Basel, Switzerland
Nowadays lighting projects often include temporal variations of the light, both spectrally
and in terms of intensity to consider non-visual effects of light on people. However, as
of today there are no specific regulations. Compliance with common lighting standards
that address visual aspects of light, often means that only little non-visually effective
light reaches the eye. In this practice review we confront existing regulations and
standards on visual lighting aspects with new recommendations on non-visual aspects
and highlight conflicts among them. We conclude with lighting recommendations that
address both aspects.
Keywords: lighting, workplace, standards, circadian rhythms, non-image forming effects of light
INTRODUCTION
The advent of electric lighting made it possible to decouple working hours by means of shift work
from times when daylight was available. Concomitant to alterations in working hours, sleep-wake
times are very often irregular in shift workers thereby impacting on the endogenous circadian
timing system with negative health consequences (1). Light as the principal synchronizer (i.e.,
Zeitgeber) of human circadian rhythms, is “seen” during the biological night while at work, which
might lead to circadian rhythms disturbance such as shift work sleep disorder (SWD). SWD is
a circadian rhythm sleep disorder characterized by insomnia and excessive sleepiness affecting
people whose work hours typically occur during the habitual sleep period (1). In particular light
with high proportions of short wavelengths in the blue spectral range in the evening and at night
suppress the secretion of the night hormone melatonin, a marker of circadian rhythmicity in
humans. Additionally to negative light effects during the night, low illuminances during the day
can destabilize circadian rhythms (2).
Continuously disturbing the entrainment of endogenous circadian rhythms with external
diurnal Zeitgeber rhythms with can weaken the regenerative ability of the organism (3,4). Night
work is associated with negative consequences for somatic and mental health (5) and persistent
desynchronization of endogenous rhythms limits cognitive performance (6). Thus, with the
increase in flexible working times, innovative lighting concepts that take particular account of
non-visual lighting effects become particularly important.
With the increased time spent in buildings, the length of time during which people are exposed
to high amounts of daylight decreases. Although, lighting standards for workplaces ensure that
we can see well, specifications for artificial lighting correspond to twilight conditions outdoors (7).
500 lx of artificial light indoors corresponds to 0.5% of the light on a cloudless day. Measurements
Stefani and Cajochen Practice Review on Existing Lighting Recommendations
at workplaces have shown that workers are usually exposed to
illuminance of only 100 lx for more than 50% of the day (8),
which fall far below the recent recommendation of daylight
illuminance exposure (9).
VISUAL EFFECTS OF LIGHT –
REGULATIONS, STANDARDS, AND
ENERGY ASPECTS
Where life safety is threatened (e.g., in emergency situations
like building fires) interior lighting projects must comply with
requirements imposed by regulations regarding minimum
levels of illuminance. Different regions of the world approach
regulations, recommended practices, and standards differently.
For the design and operation of workplaces, the German
“Technical Rules for Workplaces” (ASR) for example, reflect
the state of the art for occupational medicine, occupational
hygiene, as well as other reliable ergonomic findings for
setting up and operating workplaces. According to the German
Workplace Ordinance, workplaces must receive as much
daylight as possible and be equipped with artificial lighting
that is appropriate for the safety and health protection of
employees. The Technical Rules for Workplaces ASR A3.4
(10) “Lighting” specify the requirements of the Workplaces
Ordinance for setting up and operating the lighting in
workplaces as well as the requirements for glare protection
when exposed to sunlight. In Germany for example, an artificial
lighting system for workplaces must meet requirements
concerning illuminance, limitation of glare, color rendering,
flickering, or pulsation as well as shadows. DGUV Informative
Publication 215-210 “Natural and Artificial Lighting of
Workplaces” of the German Social Accident Insurance (DGUV)
offers also assistance to employers in implementing the
ASR A3.4 “Lighting.”
Standards differ from regulations. In general, lighting
professionals are expected to appraise each design situation
and develop criteria for illuminance, color rendering quality,
uniformity, correlated color temperature (CCT) etc., that are
appropriate for a project, and although, there is no obligation to
comply to standards, they provide valuable guidance. Workplace
lighting in particular should consider these standards. One
example is the German DIN EN 12464-1:2011-08 (11). This
standard specifies the planning principles for lighting systems,
but does not specify the requirements for the safety and
health protection of employees at work. This standard gives
illuminance recommendations for the task area, the immediate
surroundings, background area, and for walls and ceilings.
In a new draft (prEN 12464-1:2019), higher illuminances are
recommended in order to allow for adjustments. In the new
draft, the horizontal illuminance of 500 lx for workplaces e.g.,
is now specified as a minimum requirement. A higher value
(1000 lx) is specified, which is to be used e.g., in rooms with
elderly persons with lower eyesight. The demands on lighting
quality are determined by the visual tasks that the human eye
has to master. The classic quality features of lighting can be
divided into three basic quality features, which are weighted
differently depending on the use of the room and the desired
appearance: Visualization, visual comfort, and visual ambience.
The following applies:
Visual performance is influenced by the illuminance and the
limitation of direct and reflected glare.
Good color rendering and a harmonious brightness
distribution ensure visual comfort.
Visual ambience is determined by CCT, light direction, and
modeling (i.e., the distribution of light and shadows).
Good lighting systems are also characterized by energy efficiency.
However, according to DIN EN 12464-1 (11), the quality of light
should not be reduced for lower energy consumption.
The purpose of the Swiss SIA 2024 (12) is to standardize
assumptions about room usage, in particular about personal
occupancy, and equipment usage. These assumptions should
be applied in calculations and verifications according to the
standards of energy and building services engineering if there is
no more accurate information available. The requirements are
regarded as standard values for the design of plants or factories
in an early planning phase. Finally, typical values are given
for power and energy requirements in the areas of appliances,
lighting, ventilation, etc.,
Further, important features are flicker-free lighting and the
possibility of changing brightness and CCT. Luminaires that are
too bright in the field of view can cause glare. Therefore, light
sources must be shielded in a suitable way. Glare is a complex
topic that cannot be discussed in rigorous detail in this work.
Other work, such as the DIN EN 12464-1 should be referred to.
When working with PC screens, care should be taken that
the luminance ratio between the working field and its immediate
surroundings is no greater than 3:1. The luminance ratio between
the work surface and the more distant surfaces should not
exceed 10:1. Due to higher luminances and improved anti-
reflective coatings, modern PC screens can tolerate much higher
environmental luminance levels than their predecessors. DIN
EN 12464-1 describes the permissible limit values for avoiding
reflected glare. For screens with a luminance of L 200
cd/m2luminances of up to 1500 cd/m2are permissible for
luminaires. For monitors with a monitor luminance L >200
cd/m2(typical for offices with good to very good daylight supply
and correspondingly adapted flat screens) luminance values of up
to 3000 cd/m2are permissible. The ambient contrast ratio (A-
CR) is a key metric to achieve a high image quality of displays
when considering bright ambient lighting (13). While OLED
(organic light-emitting device) based displays exhibit several
attractive features, such as self-emission, high brightness, and a
high contrast ratio, when operated under bright ambient light,
most of the incident light is reflected and decreases the A-CR
which makes the application under high illuminances difficult.
Today there are several methods to eliminate reflected ambient
light in OLEDs (e.g., by a circular polarizer or a destructive
interference layer). A-CR is generally defined as
ACR =Lon +Lambient ·RL
Loff +Lambient ·RL
Frontiers in Psychiatry | www.frontiersin.org 2May 2021 | Volume 12 | Article 652161
Stefani and Cajochen Practice Review on Existing Lighting Recommendations
where Lon (Loff ) represents the on-state (off-state) luminance
value of an LCD or OLED, and Lambient is ambient luminance
(14). RLis the luminous reflectance of the display panel.
For a balanced luminance in the room all surfaces must be
taken into account. Surface luminances can be determined by the
reflectance of the surfaces and the illuminance on the surfaces.
According to DIN EN 12464-1 recommended reflectances are:
ceiling: 0.7 to 0.9
walls: 0.5 to 0.8
floor: 0.2 to 0.4
Regarding good color rendering it is commonly recommended
to have a CRI of Ra >80. We have recently found evidence for
positive effects of a high CRI (Ra 97 vs. 80) on visual comfort,
daytime wakefulness, well-being, and nighttime sleep (15). A
newer and better system for evaluating a light source’s color
rendering property is IES TM-30-15. The fidelity index and the
gamut index of IES TM-30-15 of the LED causing the before
mentioned positive effects were Rf.97 and Rg.101, respectively,
and for the poorer performing LED it was Rf.81 and Rg.94.
Without shadows objects are only two-dimensional images.
Only the correct distribution of light and shadow guarantees
that faces and gestures, surfaces, and structures can be easily
recognized (11). A pleasant lighting climate is created when
people, architecture, and room furniture are illuminated in
such a way that shapes and surface structures are clearly
visible. Distances can be easily estimated and orientation
in the room is made easier. Good visual communication
requires that faces are easily and quickly recognized. In
areas where good visual communication is important, for
example in offices and meeting areas, DIN EN 12464-1
recommends a higher average cylindrical illuminance of 150 lx.
The cylindrical illuminance is the average of all vertical light
on an imaginary cylinder. DIN EN 12464-1 cites “modeling”
as an important quality feature for the perception of people
and objects. Modeling is the relationship between cylindrical
and horizontal illuminance and should be between 0.30
and 0.60.
The international WELL Building Institute (16) aims for
advancing health and well-being in buildings. It also provides
recommendations for lighting to support visual acuity and is
mainly based on the American National Standards Institute
(ANSI) and Illuminating Engineering Society (IES) RP-1-20 (17)
standard and on the standard of the Ontario Ministry of Labour,
Computer Ergonomics: Workstation Layout and Lighting (18).
At workstations or desks requirements are met when
1. The ambient lighting system is able to maintain an average
light intensity of 215 lx or more, measured on the horizontal
work plane. The lights may be dimmed in the presence of
daylight, but they should be able to independently achieve
these levels.
2. The ambient lighting system is zoned in independently
controlled banks no larger than 46.5 m² or 20% of open floor
area of the room (whichever is larger).
3. If average ambient light is below 300 lx, task lights providing
300 to 500 lx at the work surface are available upon request.
The American National Standards Institute and Illuminating
Engineering Society of North America. RP-1-20 provides
recommended luminance ratios for offices:
1. Luminance ratios should not exceed 3:1 between a paper task
and an adjacent visual display terminal.
2. For ceiling luminance ratios, 10:1 is the maximum
acceptable ratio.
3. Luminance ratios should not exceed 10:1 between a task and a
remote surface.
In the latest version of the WELL v2 pilot (Q1 2021) it is
recommended that indoor spaces should comply with one
of the lighting reference guidelines (IES Lighting Handbook
10th Edition, EN 12464-1: 2011, ISO 8995-1:2002(E) (CIE S
008/E:2001), or GB50034-2013).
NON-VISUAL EFFECTS OF LIGHT
It is only known since 2002 that the human eye has a third
photoreceptor for processing ambient light in addition to the
two classical photoreceptor types, the rods, and cones (19,20).
Effects of visible radiation, which are mainly controlled by this
newly discovered photoreceptor, but make a minor contribution
to classical visual information processing, are also called non-
visual light effects. Berson and colleagues reported that those
ganglion cells containing the photopigment melanopsin project
almost exclusively to the nucleus suprachiasmaticus (SCN), the
central pacemaker driving circadian rhythms. They are also
able to transmit light signals into the cortex without the help
of the classical photoreceptors. Thus, these ganglion cells have
been termed “intrinsically photosensitive retinal ganglion cells
(ipRGC)” with melanopsin maximally sensitive to visible short-
wave radiation (21).
Provencio et al. (22) reported that a coarsely resolved network
of photosensitive ganglion cells extends over the retina of mice,
which has the task of detecting brightness. Later, it was found that
these melanopsin-containing ganglion cells are distributed not
only in the fovea but also over the entire retina with a density of
3–5 cells/mm² and have their maximum concentration of 20–25
cells/mm² in the area surrounding the fovea (20,23). IpRGCs are
not evenly distributed across the retina but have a higher density
in the lower half so that light that falls into the eye from above
and impinges the lower half of the retina suppresses the nocturnal
release of melatonin more than light from below (24,25).
While blue-enriched light can contribute to increased
alertness, especially in the evening and at night, nocturnal
exposure to light reduces melatonin secretion. Melatonin
secretion is especially reduced by light at night if people are
exposed to only a low dose of light during the day (26). Today,
people often live their lives in isolation from their natural
environment with a high risk to develop circadian disorders
(27). Severe circadian disturbances are found in people who are
forced to chronically change their lifestyle by adapting their sleep-
wake schedules to the imposed work schedules (e.g., rotating
shift workers) (1). A milder form of circadian disorder occurs
in many people due to too short night’s sleep on working days
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Stefani and Cajochen Practice Review on Existing Lighting Recommendations
and a delayed onset and prolonged duration of sleep on non-
working days. Over 80% of people in western industrialized
countries show this altered sleep behavior and that nightly
sleep on non-working days is delayed by an average of 90
min (28).
Since circadian rhythms only have an approximate period
length of 24 h, they require daily synchronization with the
environment (29,30). The rotation of the earth and thus the
regular light-dark change is the most important environmental
signal for the synchronization of circadian rhythms (31).
For the interpretation of ambient brightness, photoreceptors
continuously calculate the intensity, and spectral composition
of the light entering our eyes (32). Thus, when light with
increased short-wave radiation enters the eye, ipRGCs signal
a bright phase of day to the SCN. In addition, the times of
change from dark to light phase and vice versa (i.e., dawn
and dusk) provide a crucial input for the SCN and the
synchronization of circadian rhythms with the environment
(31,33,34). Bright white light at night can shift the
circadian phase backward by up to 3 h in the next 24-h
cycle. In contrast, early morning exposure to light can shift
the circadian phase forward by up to 2 h in the next 24-h
cycle (35,36).
Some studies document acute effects of bright light on the
subjective feeling of alertness at night and during usual sleep
periods (3739), before falling asleep (40) and immediately
after waking up in the morning (41,42) but also during
daytime (43). Although, most of these studies compare very low
illuminances (5–50 lx) with high illuminances (1,000–5,000 lx)
of fluorescent white light, alerting effects were estimated to
occur already at around 100 lx during the night and at 500 lx
during the evening. Thus, there is evidence, that subjective
wakefulness due to bright light can basically occur at any
time of the day. Other research with objective measures
has so far only been able to prove a wakefulness inducing
effect of bright light at night (44) and results of studies
conducted during the day provided inconsistent results (43,
45,46), possibly due to smaller differences in the light levels
compared. Nevertheless, bright light during the day makes
the circadian system less sensitive to nocturnal light (26,47
49). Although the protocols of these studies differ from each
other, illuminances that were compared during the day were
significantly different from each other (i.e., at least 10-fold
up to 400-fold different). A dark phase of several hours can
increase sensitivity to light. Thus, early morning exposure to light
immediately after awakening (i.e., after several hours of nightly
sleep in darkness) can phase advance the circadian phase by
1–3 h (50,51).
In many cases, the alerting effect of light, especially in the
evening and at night, is also associated with an increase in
attention and working memory performance (41,52). Non-
visual effects also include the mood enhancing effect of bright
light (5355), with required light doses being around 2500
lx-h. There is also evidence that the current mood reflects
a person’s level of alertness and the immediate effect of
bright light on mood is mediated by the wakefulness inducing
effect (56).
LIGHTING CONCEPTS THAT ADDRESS
NON-VISUAL EFFECTS OF LIGHT
Considering our experience with around 100 years of electric
lighting, people have been exposed to this artificial creation 5,000
times shorter than to the light at night from fire. The first
evidence for the use of handcrafted light sources comes from
archaeological findings from around 500,000 years ago. Fire was
used as a source of light at night, but life and work still depended
on daylight. Daylight is perhaps the purest form of human centric
lighting since our eyes have had several million times longer to
optimize to daylight than to LEDs. It is therefore reasonable to
assert, from the evolutionary standpoint, that human eyes, and
behavior are not yet optimized to electric light.
New lighting technologies that try to mimic continuously
changing CCT and illuminance of sunlight according to the
time of day are often termed HCL (Human Centric Lighting).
According to manufacturers of these lighting technologies,
it is possible to provide people indoors with artificial light
similar to daylight in such a way that they can benefit from
the beneficial effects natural daylight would provide. These
include increased alertness, concentration, and performance. A
publication summarizing the benefits of HCL on humans shows
that it has sound motivations (57). The authors conclude that
“bright days and dark nights are a good starting point,” and
suggest that apart from electric lighting, architecture should
be driven by daylight design principles. A conclusion that we
support. Since HCL is increasingly being promoted and used
for work places or private homes due to their postulated effects,
the Swiss State Secretariat for Economic Affairs (SECO), and
the Federal Office of Public Health (FOPH) have commissioned
the Centre for Chronobiology at the University of Basel to
evaluate scientific literature on HCL (58). The central question
was whether this light can influence physiological, cognitive,
or subjective effects in humans, i.e., the effects perceived by
humans themselves.
The Basel study (58) has shown that only a few studies have
investigated whether HCL can influence the above mentioned
effects. Therefore, the University of Basel has extended the
assessment and additionally evaluated studies on physiological,
cognitive, or subjective effects of artificial light that affects people
during the day during office hours (from 7:00 a.m. to 5:00 p.m.)
but does not continuously adapt to the properties of daylight. A
total of 45 studies met the inclusion criteria. On the basis of these
studies, it was possible to check for 33 different effect variables
whether they depend on the light intensity and CCT of artificial
light that affects people during the day.
The Basel study (58) shows that neither the light intensity
nor CCT significantly influence physiological parameters such as
pulse rate and brain waves during normal office hours. In the case
of cognitive effects, however, it was shown that light intensity
and CCT had an influence on the reaction time of people. In
addition, the spectrum of light influences the accuracy with
which people solve tasks. In the subjective effects, light intensity
and light spectrum had an influence on the concentration,
tiredness and drowsiness perceived by the persons themselves.
Overall, however, the observed effect strengths of the light effect
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Stefani and Cajochen Practice Review on Existing Lighting Recommendations
during office hours were rather small. Nevertheless, the authors
of the study come to the conclusion that high light intensity
and higher CCT during daytime hours are advantageous in
artificially illuminated interiors, even if these advantages are only
evident in cognitive and subjective effects but not in physiological
parameters. During the night, the effects of higher CCT are more
prominent even in field studies. While blue-enriched white light
sources can adjust circadian rhythm to night-shiftwork, reduce
sleepiness, and enhance cognitive performance of night-shift
workers (59) this concept should be applied cautiously and only
if workers must work very concentrated (e.g., in control rooms).
While there are numerous studies providing evidence of non-
visual effects of light during the evening and at night, results
might not be translatable to the day. In line with the Basel study
(58), a literature review on daytime non-visual effects of light
on alertness (60) concludes that the present literature provides
inconclusive results on alerting effects of light during daytime,
particularly for objective measures and correlates of alertness.
The authors suggest that the alerting potential of exposure to
more intense white light should still be investigated. Another
systematic review assessed effects of light on alertness and mood
in daytime workers (61). Although, they conclude that light with
a high CCT may improve alertness during the day, they suggest
that additional studies are still needed because all findings are
based on low-quality evidence.
Impacts of two dynamic LED lighting concepts with
illuminance and CCT gradually decreasing between 1:30 p.m.
and 5 p.m. were investigated on its effects on sleep and well-
being (62). In one setting illuminance changed from 700 to 500 lx
and CCT from 6000 to 3500 K, in the other from 500 to 300 lx,
and CCT from 5000 to 3000 K. The settings were compared
to static light (500 lx, 5000 K and 300 lx, 4000 K). A significant
increase in subjective alertness was observed at 1 p.m., indicating
a potential solution to reduce the subjective sleepiness in the
afternoon. On the other hand, a significant decrease in perceived
sleep quality and sleep duration was reported after subjects
were exposed to dynamic lighting. No significant differences
were observed for mental stress, productivity, visual comfort, or
perceived naturalness.
A different approach with custom-built desktop luminaires
intended to support office occupants’ entrainment while
supporting their alertness during the day. The luminaires
were designed to deliver three lighting interventions. First
saturated blue light (455 nm, 50 lx) in the morning (6–12 a.m.),
Then polychromatic white (6500 K, 200 lx) light at midday
(12 a.m.1:30 p.m.) provided a smooth transition from the first
to the third intervention. The third intervention was saturated
red light (634 nm, 50 lx) in the afternoon (1:30–5 p.m.). In their
results the authors observed advances in sleep start and sleep end
times and hence they suggest that the participants were better
entrained to the local 24-h light dark cycle while at the same time
reporting increased subjective alertness in the afternoon with red
light (63).
The effect of dynamic light during shift work on the quality
of sleep and melatonin secretion was examined with staff of an
Intensive Care Unit (ICU) and compared with staff from a similar
ICU with standard light (64). CCT controlled ceiling luminaires
with light tubes (2700 and 6500 K) and indirect lighting with
RGBW lights “to imitate the reflection of the sun” were used
but no information about the spectral characteristics is reported.
The light changed color and intensity. The nightlight between 10
p.m. and 5 a.m. was dim (68 lx), and short-wavelength depleted
causing the light to appear “unnaturally red.” Between 5 and
6 a.m., the light gradually changed to a daylight scenario (525 lx).
In the afternoon from 3 p.m. light levels decreased. Between
8 and 10 p.m. a change occurred “toward a mix of primarily
red, green, and white.” Since no precise spectral measurements
are available but only RGB percentages it is difficult to replicate
the lighting conditions. Nevertheless, the intervention group
reported to be more rested and assessed their condition on
awakening as better than the control group. The study, however,
found no significant differences in sleep efficiency and melatonin
levels. Subjectively, nurses from the intervention group assessed
their sleep as more effective than participants from the control
group. In a different field study, bright fluorescent lighting
(1500–2000 lx) when compared to standard lighting (300 lx) in
hospitals decreased sleepiness of ICU nurses working a 10-h
night shift (65).
In a field experiment, effects of dynamic lighting on office
workers were tested (66). In the dynamic lighting condition,
employees experienced a gradually changing lighting scenario
(changing twice a day between 8 and 12 a.m. and 1:30 and 4 p.m.
from 700 to 500 lx and 4700 to 3000 K). The static condition
provided an illuminance of 500 lx and CCT of 3000 K While
employees were more satisfied with the dynamic lighting there
were no significant differences for need for recovery, vitality,
alertness, headache and eyestrain, mental health, sleep quality, or
subjective performance.
Under strictly controlled laboratory conditions, we examined
whether dynamic light across the day influences cognitive
performance, visual comfort, melatonin secretion, sleepiness,
and sleep (67). Volunteers either woke up with static daylight
LED (100 lx at the pillow and 4000 K, melEDI 69 lx) or with a
dynamic daylight LED that changed CCT (2700–5000 K) and
intensity (0–100 lx at the pillow, melEDI 0.4–76 lx) across the
day (daylight here refers to the spectral characteristics of the
Toshiba TRI-R LED). Participants underwent a 49-h laboratory
protocol. They spent the first 5-h in the evening under standard
lighting, followed by an 8-h nocturnal “baseline” sleep episode
at habitual bedtimes. Thereafter, they spent a scheduled 16-h
waking day under one of the lighting conditions. Following a
8-h nocturnal “treatment” sleep episode, the volunteers spent
another 12 h either under static or dynamic light. Horizontal
illuminance at desk height ranged, depending on the position,
between 150 and 650 lux, which corresponds to standard office
lighting. Under dynamic light, evening melatonin levels were less
suppressed 1.5 h prior to usual bedtime, and participants felt less
vigilant in the evening compared to static light. Sleep latency
was significantly shorter compared to the static light condition
while sleep structure, sleep quality, cognitive performance,
and visual comfort did not significantly change. These results
support the recommendation of using blue-depleted light and
low illuminances in the late evening, which can be achieved
by a dynamically changing LED solution. Since illuminance
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Stefani and Cajochen Practice Review on Existing Lighting Recommendations
decreased to around 1 lx, this lighting concept can only be
applied in domestic areas but if concentration at work in the
late evening and at night is required, this lighting concept would
be counterproductive.
To date, only a few field studies investigated the influence
of dynamic lighting solutions during shift work. A field study
surveying the state of subjective alertness and fatigue in 542
employees during three-shift work (8 h working shifts) in
ongoing production operations compared dynamic light with a
static lighting condition (68). In a first round, 256 respondents
evaluated the static lighting concept by completing a structured
questionnaire. In a second round, 287 respondents commented
on the alternating lighting concept. Fourty one percent of the
participants who experienced the alternating lighting concept
took part in the survey on the static lighting concept. They
worked in three shifts (morning, late, and night) for 8 h each.
The alternating lighting concept featured a high horizontal
illuminance on the workplace (850 lx) and a high CCT (5300 K)
during daytime. The resulting vertical illuminance at eye height
was 237 lx (melEDI 164 lx) and CRI was Ra 77. During nighttime,
a reduced illuminance (580 lx) with low CCT (3400 K) and CRI
of Ra 85 was deployed. This resulted in a vertical illuminance
of 158 lx (melEDI 71 lx) at eye height. Illuminance and CCT
changed gradually between 5 and 8 p.m. and between 5 and
9 a.m. This setting was compared to the static lighting condition
(horizontal at the workplace 760 lx, vertical at the eye 210 lx
(melEDI 128 lx), 4600 K, CRI Ra 82). All participants assessed
specific lighting characteristics (such as CCT, brightness, color
rendering, appeal) using a seven-point Likert scale. No significant
differences were found between the participants’ rating of the
characteristics surveyed in terms of alternating and static lighting
conditions. The transitions from day to night conditions and
vice versa had no disturbing effect on the participants’ rating of
the lighting criteria surveyed (p>0.05). Thus, it was concluded
that shift workers accepted the alternating lighting system.
In addition, there were no significant differences in alertness
and fatigue between the early and late shifts for both lighting
conditions. This survey indicated the potential usefulness of a
dynamic lighting solution for shift-work without major impact
on the worker’s alertness and fatigue level. However, objective
measures such as salivary melatonin levels, and reaction time
measures to assess vigilant attention are clearly mandatory for
future studies to test the usefulness of dynamic lighting solutions
in shift work environments.
With the goal to provide adequate light for visual tasks while
lessening disruption of the human circadian system, Moore-
Ede et al. (69) derived a spectral sensitivity curve with a peak
at 477 nm and a full-width half-maximum of 438 to 493 nm.
While there are other products commercially available that notch
the spectrum in the melanopsin region, they specifically call it
“steadystate circadian potency spectral sensitivity” and suggest
that it “permits the development of spectrally engineered LED
light sources to minimize circadian disruption and address the
health risks of light exposure at night in our 24/7 society, by
alternating between daytime circadian stimulatory white light
spectra and nocturnal circadian protective white light spectra.”
They further suggest, that it could provide attractive and energy-
efficient white electric light that minimizes circadian disruption
if violet LED dies with peak wavelengths of 410 to 420 nm
replace the typical 450 nm blue peak emissions of conventional
LEDs. Since short-wavelength light is known to have alerting-,
performance-, and mood enhancing properties (70,71), they
suggest that this light at night could be used to reduce human
error, without the risk of circadian disruption and health
disorders. The alerting effect of short-wavelength light could be
retained because there is evidence by a single study that the
alerting effects of 420 nm violet light are even greater than 440
or 470 nm blue light (72).
In an approach to mimic certain aspects of daylight (i.e., direct
warm sunlight and diffuse cool skylight), Aalborg University
proposed a combination of directional task lighting and diffuse
ambient lighting, with respective intensities and CCTs to create
naturally perceived luminous variations. Such lighting concepts
mimicking the combination of light from the sun and sky date
back to 1952 (73). Aalborg University conducted a pilot study
with four participants that worked for 4 months in such static
and dynamic lighting. Visual comfort, perceived atmosphere,
and work engagement were evaluated with interviews and
questionnaires. The tentative results indicate that dynamic
lighting has a positive effect on visual comfort, perceived
atmosphere, and work engagement compared to static lighting
(74). Another study (75) from the same author investigated
the quality of light in an office after adding ceiling-mounted
spotlights to traditional diffuse ceiling panels with the intention
to complement the directionality of the natural daylight inflow
from windows. The visual light quality and perceived atmosphere
of the office environment was tested with 30 volunteers through
questionnaires, reaction cards and semi-structured interviews.
The authors report: “The direct flow of light is recommended
to be more than 15% of the total illuminance at the work-plane
to provide the distinct visual appearance of modeling and a
cozier atmosphere, which is preferable for socializing, and <45%
to avoid glare and high contrast for visual tasks. Direct warm
and diffuse cool lighting were perceived as the most natural but
were not always preferred. There is a slight preference for cooler
ambient lighting in clear sky situations and warmer ambient
lighting in overcast situations. Strong individual preferences for
combinations of color temperatures was identified. . . .”
Effects on well-being and motivation by changing light
distribution were investigated by Fleischer (76) in 2001. Lighting
consisted of luminaires that slowly changed between direct and
indirect light. The ratio between direct and indirect lighting
was changed according to the time of the day or to weather
conditions. It was shown that pleasure rises with higher
illuminance and a large indirect component. This might be due
to a “sky-like” impression of the bright ceiling. The preference
for a large indirect component was also found by Houser et al.
(77) who report a subtle overall preference, when the indirect
contribution to horizontal illuminance was 60% or greater.
With an increase of the direct component and an increase of
illuminance arousal rises. The direct component results in a
darker ceiling but brighter desk. Apparently this contradicts the
findings of (23) and (25) that find higher sensitivity of ipRGCs
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Stefani and Cajochen Practice Review on Existing Lighting Recommendations
for light coming from above. The results of Fleischer, however,
may not be explained by NIF effects (by ipRGCs) but solely by
visual effects.
EXISTING RECOMMENDATIONS FOR
NON-VISUAL AND CIRCADIAN ASPECTS
From a non-visual and circadian perspective, compliance with
the above-mentioned standards cannot guarantee that enough
biological active light reaches the eye (78). Some studies (45,79,
80) report that corneal illuminance levels of at least 1000 lx for
several hours are necessary to achieve non-visual effects during
the day. Many of these studies investigated the effects of light
therapy in the morning while others also found effects with
illuminances ranging from 1000 to 1700 lx during various normal
office hours (when compared to 165–200 lx). Thus, lighting
standards addressing visual aspects are currently not designed to
account for non-visual light effects during the day.
Today, an evaluation of the non-visual effectiveness of
radiation is based on a radiometric characterization of the
radiation entering the eye, and the corneally measured spectral
irradiance is weighted with the spectral sensitivity of all five
photoreceptors and referenced to the spectrum D65 (standard
illuminant) (81). The International Standard of the International
Commission on Illumination (CIE) CIE S 026:2018 (82) “CIE
System for Metrology of Optical Radiation for ipRGC-Influenced
Responses to Light” defines spectral sensitivity functions,
quantities, and metrics to describe the ability of optical radiation
to stimulate each of the five photoreceptor types (S-cone, M-
cone, L-cone, rhodopsin, and melanopsin). This standard also
denotes a quantity named the “melanopic equivalent daylight
illuminance” (melanopic EDI or melEDI), that is expressed in
Lux. The melanopic EDI of a light condition expresses how much
daylight results in the same melanopic irradiance as the test light
condition. Nowadays lighting projects often include temporal
variations of the light, both spectrally and in terms of intensity.
Lighting projects that consider the possible effects of changing
light on people, try to optimize well-being. However, as of today
there are no specific regulations. Recommended practices sprout
everywhere but experts in the field criticize them.
Seven examples for recommendations for circadian lighting
are (in alphabetical order):
1. Chartered Institution of Building Services Engineers (CIBSE)
and Building Research establishment (BRE) Research Insight
Circadian lighting (83).
2. CIE S 026:2018 (82) “CIE System for Metrology of Optical
Radiation for ipRGC-Influenced Responses to Light.”
3. DGUV 215-210 “Non-visual Effects of Light on Humans”
(84).
4. DIN SPEC 67600:2013-04 (85) (technical report) “Biologically
Effective Lighting - Planning Recommendations.”
5. Recommendations for Healthy Daytime, Evening, and Night-
Time Indoor Light Exposure (9).
6. UL DG 24480 (86) “Design Guideline for Promoting
Circadian Entrainment with Light for Day-Active People.”
7. The WELL standard “CIRCADIAN LIGHTING DESIGN”
and the update WELL v2 pilot (87).
CIBSE and BRE Research Insight Circadian
lighting
Based on a literature review (83) and their results from a field-
study they suggest these tentative recommendations:
1. “From mid-morning until early afternoon, use higher than
normal levels of light with increased blue light. Current high
color temperature light sources such as LEDs and some types
of fluorescent light give high outputs of blue light. There is still
scope to tailor their spectra further in the future to fit the peak
response of the ipRGC sensors in the eye and maximize their
circadian impact.
2. Toward the end of the day, dim the lighting (while retaining
enough light to meet visual task recommendations) and lower
its color temperature (“warmer,” redder light, similar to that
in a domestic setting). There is also future scope to alter the
spectrum of existing LEDs to give very low circadian stimulus
in the evening or at night. Even warm white LEDs often have a
small peak of blue light which can stimulate the ipRGCs.
3. Maximize reflected light from room surfaces by using light
fittings with an upward light component, and “wall washing”
to illuminate the walls directly. This will give more light to
people facing the walls.
4. As light levels will be higher than normal for part of the day,
use high quality fittings to minimize glare and avoid all flicker.
Have a balanced visual environment, for example by avoiding
very light-colored desks.
5. Vary the lighting gradually, to avoid disturbing the occupants.
Controls need to be reliable.
6. People vary in their preferences for lighting; conventional
good practice is to offer individual control but this can negate
the circadian effects. There is no obvious way round this.
7. Explain to the occupants what the lighting system is doing and
the purpose of varying the lighting.”
CIE S 026:201861 “CIE System for
Metrology of Optical Radiation for
ipRGC-Influenced Responses to Light”
CIE recommends to spend adequate time outdoors during the
day since it is associated with better health and well-being and
also recommends to not restrict daylight within indoor settings.
Although, no specific quantities are given, CIE recommends a
high melEDI during the day to support alertness, the circadian
rhythm, and good sleep during the night in a position statement
on Non-Visual Effect of Light. During the evening and at night a
low melEDI facilitates sleep initiation and consolidation (88).
DGUV 215-210 “Non-visual Effects of Light
on Humans”
This DGUV (German statutory accident insurance) information
brochure (84) provides advice on hazards to safety and health
at work, how they can be avoided and how opportunities
for maintaining health can be exploited with modern lighting
concepts. Since scientific knowledge about the non-visual effects
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Stefani and Cajochen Practice Review on Existing Lighting Recommendations
of light on humans is not yet complete, as the brochure says,
it is not yet possible to derive any generally valid quantitative
statements regarding non-visual effects, for example numerical
values for illuminance or CCT. This brochure gives the advice
that daylight should be used first and foremost. For this reason,
workplaces should preferably be located close to windows. The
better the inner clock is synchronized by daylight, the less
sensitive it is to disturbing factors, such as artificial light in
the evening. Only if little daylight is available at workplaces,
bright artificial lighting or lighting with high blue components
should be used as a supplement during the day. Light sources
with high CCTs are usually favorable for this purpose. This light
can achieve similar non-visual lighting effects as daylight, but
cannot replace it. In the evening, bright light and light with
high blue components should be avoided. This should be done
at least 2 h before the usual start of sleep. During this time,
the light should primarily illuminate the work surface relevant
to the visual task and not fall directly into the eye. Looking
directly into the light source and at very brightly illuminated
surfaces should be avoided. When working on a computer, tablet
or smartphone, special blue light filter programs (e.g., flux, Night
Shift or other manufacturer-specific blue light filter apps) should
be used at least 2 h before the usual start of sleep. Furthermore,
advice is given, that during the day, bright walls, and ceilings
should enhance the non-visual effects through indirect light
components. In the evening, the necessary brightness at the
workplace should be limited. The lower indirect share of light on
the ceiling and walls should reduce non-visual effects.
DIN SPEC 67600:2013-04 “Biologically
Effective Illumination - Design Guidelines”
The German DIN SPEC 67600:2013-04 “Biologically effective
illumination - Design guidelines” recommends: Illuminance at
the eye 250 lx at CCT =8000 K or Illuminance at the eye
290 lx at CCT =6500 K. The Commission for Occupational
Health and Safety and Standardization (KAN) (represents
occupational health and safety interests in the standardization
process) criticizes (89):
“Contents of the already published DIN SPEC 67600:2013-04
(technical report) “Biologically Effective Lighting - Planning
Recommendations” are partly based on insufficiently secured
findings, therefore a misinterpretation during its application
cannot be excluded . . . the planning recommendations of DIN
SPEC 67600 (technical report) do not form a secure basis for
the implementation of the Technical Regulation for Lighting ASR
A3.4 in operation.”
CIBSE and BRE concluded in a literature review: “The
existing recommendations in DIN SPEC 67600 should be treated
with caution.”
Recommendations for Healthy Daytime,
Evening, and Night-Time Indoor Light
Exposure
A recent publication (9) from experts in lighting,
neurophysiological photometry and sleep and circadian
research provides an expert consensus for healthy daytime
and evening/night-time light environments. They come to the
conclusion that “Throughout the daytime, the recommended
minimum melEDI is 250 lx at the eye measured in the vertical
plane at 1.2 m height (i.e., vertical illuminance at eye level when
seated). If available, daylight should be used in the first instance
to meet these levels. If additional electrical lighting is required,
the polychromatic white light should ideally have a spectrum
that, like natural daylight, is enriched in shorter wavelengths
close to the peak of the melanopic action spectrum. During the
evening and at home, Brown et al. (9) recommend to reduce
melEDI to around 10 lx at least 3 h before bedtime. During sleep
the recommended maximum melEDI is 1 lx.
UL DG 2448022 “Design Guideline for
Promoting Circadian Entrainment With
Light for Day-Active People”
UL DG 2448022 “Design Guideline for Promoting Circadian
Entrainment with Light for Day-Active People” recommends:
“The amount of light equivalent to that after 1 h of exposure,
capable of suppressing the production of melatonin at night by
30 per cent . . . should be continuously available at the occupant’s
eyes for a minimum of 2 h during the daytime.” This would
translate into a vertical illuminance at the eye of about 350 lx for
warm light (CCT <3000 K) and 200 lx for cool light (CCT >
5000 K) sources. Here the question arises, how a suppression of
melatonin at night relates to a measure of light during the day.
UL DG 2448022 is commented by the IES (90) as follows:
“It is important to note that UL Design Guideline 24480 is not a
consensus (ANSI) document. The IES maintains the position that
any Recommended Practice related to light and health should be a
consensus document developed through an accredited American
National Standards Institute process. Without the full rigor of
an ANSI approved Standard, non-consensus based information
cannot be deemed to have been fully vetted and lacks the authority
to provide public guidance regarding means or methods that
affect public health. The IES urges the lighting industry to exercise
caution when considering a non-consensus document for design,
application, product qualification or regulatory purposes.”
The Well Standard and Well v2 Pilot
The WELL standard recommends for melanopic light intensity at
work areas: “Light models or light calculations demonstrate that
at least one of the following requirements is met”:
1. At 75% or more of workstations, at least 200 equivalent
melanopic lx (EML) is present, measured on the vertical plane
facing forward, 1.2 m above finished floor (to simulate the view
of the occupant). This light level may incorporate daylight, and
is present for at least the hours between 9:00 a.m. and 1:00 p.m.
for every day of the year.
2. For all workstations, electric lights provide maintained
illuminance on the vertical plane facing forward (to simulate
the view of the occupant) of 150 EML or greater.
The newer WELL v2 pilot recommends these levels for all spaces
(at least 150 EML) and adds the corresponding EDI value (136
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Stefani and Cajochen Practice Review on Existing Lighting Recommendations
melEDI). In case that 218 melEDI or more are achieved, the space
would achieve a better score in a system based on points. These
light levels vertical plane at the eye should be achieved at least
between the hours of 9 a.m. and 1 p.m. and may be lowered after
8 p.m. at night.
CIBSE and BRE concluded in a literature review (91): “The
existing recommendations in the WELL Building Standard should
be treated with caution.”
CONCLUSION
The circadian timing system in humans is genetically timed
in such way that we are active and awake during daytime
and inactive and asleep during the night (i.e., diurnal species).
Thus, during the biological night the hormone melatonin is
actively secreted in a circadian fashion usually peaking 2–3 h
after habitual bedtime. As melatonin is important for many
physiological processes in the human body (e.g., antioxidant
and regulating sleep-wake timing), its secretion should not be
suppressed or altered in the evening and at night by light.
Avoiding light at night during shift work, however, is rather
difficult, especially if workers need to be fully concentrated. Thus,
ideal lighting conditions during night shifts is always a trade-off
between optimal light for visual tasks, safety, alertness, and well-
being and optimal light for non-visual effects avoiding circadian
phase shifts and melatonin suppression.
Exposure to brighter light and light with high proportions
of short wavelengths in the blue spectral range during the day
can improve subjective alertness, concentration, the reaction
time, and accuracy with which individuals solve tasks. It reduces
tiredness and drowsiness and helps to maintain circadian
rhythms and improve sleep quality as compared to darker and
blue depleted light. Physiological measures (e.g., EEG), however,
are less likely to be affected by light during the day, most probably
related to the fact that we are a diurnal species. The pathway
of light for affecting circadian rhythms, sleep-wake behavior,
alertness and well-being in individuals is predominantly expected
to be through the eye and the stimulation of ipRGCs, which
then send signals to the suprachiasmatic nucleus (SCN) and
other areas in the brain implicated in the regulation of
different neurobehavioral domains. Therefore, it can be assumed
that melEDI is a suitable measure for predicting melatonin
suppression and other non-visual effects in humans. A precise
quantity of melEDI to trigger these effects however is difficult to
determine and may depend on the output domain (e.g., alertness,
melatonin, sleep, circadian phase shifts etc.,) one is interested
in. Based on the expert consensus (9) mentioned above, we
recommend to aim for 250 lx melEDI during usual daytime office
hours (in the vertical plane at 1.2 m height) for everyone working
inside buildings, even if this requires more energy.
Notably, luminous efficacy is calculated in Lumens per Watt
because Lumens are based on visual brightness perception
(V lambda curve). Non-visual aspects, however, should be
considered too. Non-visual effects have a different spectral
sensitivity curve compared to the visual perception of brightness
and are therefore not considered in the common calculation
of energy efficiency; therefore, we would recommend not only
considering Lumens per Watt as a measure for luminous efficacy
but also a measure that considers “non-visual luminous efficacy”
during the day (e.g., melanopic EDI per Watt). When light is
capable to reinforce normal circadian patterns of alertness during
the day and sleep at night, studies have either used very high light
levels (approximately 1000 lx) or strongly blue enriched light
(CCT 17000 K). Notably, most field studies have not controlled
for the position of individuals within a room and hence for their
precise light exposure levels at the eye.
A lighting concept that reduces melatonin suppression during
the night while still allowing for high concentration would be
ideal. Since melatonin suppression is mainly linked to the melEDI
(92,93) and not necessarily to CCT, metameric light sources that
reduce melatonin suppression could be an innovative solution
(94). By optimizing the light spectrum metamerically, daytime
alertness could also be improved (95). Additionally, the light
distribution could be changed between night and day. Since
direct light increases arousal (76) and iPRGCs are probably more
sensitive to indirect light from the ceiling (25), these factors could
be considered in the lighting design for day and night. Following
existing guidelines to avoid glare and yet achieving a high amount
of melEDIs at workplaces during the day could be achieved by
three workarounds:
1. Optimizing the spectrum by using light sources with a
relatively high melEDI (and a high CRI).
2. Optimizing vertical illuminances at the eye by optimizing
the light distribution (also considering the reflection of
surrounding surfaces. Optimized lighting design can provide
higher light levels at the eye (vertical illuminances) for the
same horizontal illuminance. Often, downwards oriented
lighting from ceiling mounted luminaires intended for rooms
with PC screen result in relatively low vertical illuminances.
To achieve higher vertical illuminance and hence more light at
the eyes, suspended or floor standing luminaires with indirect
light distribution (that direct light onto the ceiling) and “wall
washing” luminaires can be used. Additional white vertical
elements can increase vertical illuminances.
3. Since the melanopsin-containing ganglion cells in the eye are
distributed over a large area of the retina, it can be assumed
that the non-visual effect of light is greatest when light comes
from a large-area source. In nature, this light comes from the
sky. If only a small area of the retina is illuminated, as is the
case with the directional light of a spot, a weaker non-visual
effect is assumed.
During the evening we share the opinion with Brown and
colleagues (9) of reducing melEDI to around 10 lx, of course, only
if no safety-relevant activities have to be carried out (i.e., at home
before bedtime).
DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included
in the article/supplementary material, further inquiries can be
directed to the corresponding author/s.
Frontiers in Psychiatry | www.frontiersin.org 9May 2021 | Volume 12 | Article 652161
Stefani and Cajochen Practice Review on Existing Lighting Recommendations
AUTHOR CONTRIBUTIONS
OS wrote the main manuscript text. CC provided
critical review of and revisions to the manuscript. All
authors contributed to the article and approved the
submitted version.
FUNDING
OS disclosed receipt of the following financial support for the
research, authorship, and/or publication of this article. The study
was partly supported by Schweizerische Bundesbahnen AG,
(SBB), Switzerland.
REFERENCES
1. Cheng P, Drake C. Shift work disorder. Neurol Clin. (2019) 37:563–
77. doi: 10.1016/j.ncl.2019.03.003
2. Rosenthal NE, Sack DA, Gillin JC, Lewy AJ, Goodwin FK, Davenport
Y, et al. Seasonal affective disorder: a description of the syndrome and
preliminary findings with light therapy. Arch Gen Psychiatry. (1984) 41:72–
80. doi: 10.1001/archpsyc.1984.01790120076010
3. Tarocco A, Caroccia N, Morciano G, Wieckowski MR, Ancora G, Garani G, et
al. Melatonin as a master regulator of cell death and inflammation: molecular
mechanisms and clinical implications for newborn care. Cell Death Dis. (2019)
10:317. doi: 10.1038/s41419-019-1556-7
4. Hildebrandt G, Moser M, Lehofer M. Chronobiologie und Chronomedizin.
Stuttgart: Hippokrates (1998).
5. Amlinger-Chatterjee. Psychische Gesundheit in der Arbeitswelt -
Atypische Arbeitszeiten. Dortmund: Bundesanstalt für Arbeitsschutz und
Arbeitsmedizin (2016).
6. Ahmad M, Md Din NSB, Tharumalay RD, Che Din N, Ibrahim N, Amit
N, et al. The effects of circadian rhythm disruption on mental health and
physiological responses among shift workers and general population. Int J
Environ Res Public Health. (2020) 17:7156. doi: 10.3390/ijerph17197156
7. Lam H, Gani S, Mawson R, Young J, Potma E. A practical tool for ambient
illumination comparisons at dusk/dawn. Proc Hum Factors Ergon Soc Annu
Meet. (2016) 60:470–4. doi: 10.1177/1541931213601107
8. Hébert M, Dumont M, Paquet J. Seasonal and diurnal patterns of
human illumination under natural conditions. Chronobiol Int. (1998) 15:59–
70. doi: 10.3109/07420529808998670
9. Brown T BG, Cajochen C, Czeisler C, Hanifin J, Lockley S, Lucas R, et
al. recommendations for healthy daytime, evening, and night-time indoor
light exposure. (2020). doi: 10.20944/preprints202012.0037.v1. [Epub ahead
of print].
10. ASR A3.4 Beleuchtung. Technische Regel für Arbeitsstätten. (2011). p.
287. Available online at: https://www.baua.de/DE/Angebote/Rechtstexte-
und-Technische- Regeln/Regelwerk/ASR/ASR-A3-4.html (accessed April 29,
2020).
11. IN Deutsches Institut für Normung. DIN EN 12464-1 Light and lighting -
Lighting of work places - Part 1: Indoor work places. (2019). Available online
at: https://www.din.de/en/getting-involved/standards- committees/fnl/drafts/
wdc-beuth:din21:302583817?destinationLanguage=&sourceLanguage=
(accessed April 29, 2020).
12. SIA (2024). Space Utilization Data for Energy
and Building Technology. (2015). Available online
at: http://shop.sia.ch/normenwerk/architekt/sia%202024/d/2015/D/Product
(accessed April 29, 2021).
13. Edward F. Kelley ML, Penczek J. Display daylight ambient contrast
measurement methods and daylight readability. J Soc Inf Display. (2006)
14:1019–30. doi: 10.1889/1.2393026
14. Haiwei Chen GT, Wu S-T. Ambient contrast ratio of LCDs and OLED
displays. Opt Express. (2017) 25:33643–56. doi: 10.1364/OE.25.033643
15. Cajochen C, Freyburger M, Basishvili T, Garbazza C, Rudzik F, Renz
C, et al. Effect of daylight LED on visual comfort, melatonin, mood,
waking performance and sleep. Light Res Technol. (2019) 51:1044–
62. doi: 10.1177/1477153519828419
16. WELL. The WELL Building Standard, WELL v2, (2021). Available online at:
https://v2.wellcertified.com/v/en/light (accessed April 29, 2020).
17. Recommended Practice. Lighting Office Spaces. New York, NY: Illuminating
Engineering Society (2020).
18. Ontario Ministry of Labour. Computer ergonomics: workstation layout and
lighting. In: Health and Safety Guidelines. (2004).
19. Berson DM, Dunn FA, Takao M. Phototransduction by retinal
ganglion cells that set the circadian clock. Science. (2002)
295:1070–3. doi: 10.1126/science.1067262
20. Hattar S, Liao HW, Takao M, Berson DM, Yau KW. Melanopsin-containing
retinal ganglion cells: architecture, projections, and intrinsic photosensitivity.
Science. (2002) 295:1065–70. doi: 10.1126/science.1069609
21. Paul KN, Saafir TB, Tosini G. The role of retinal photoreceptors in the
regulation of circadian rhythms. Rev Endocr Metab Disord. (2009) 10:271–
8. doi: 10.1007/s11154-009-9120-x
22. Provencio I, Rollag MD, Castrucci AM. Photoreceptive net in the mammalian
retina. This mesh of cells may explain how some blind mice can still tell day
from night. Nature. (2002) 415:493. doi: 10.1038/415493a
23. Dacey DM, Liao HW, Peterson BB, Robinson FR, Smith VC, Pokorny
J, et al. Melanopsin-expressing ganglion cells in primate retina signal
colour and irradiance and project to the LGN. Nature. (2005) 433:749–
54. doi: 10.1038/nature03387
24. Lasko TA, Kripke DF, Elliot JA. Melatonin suppression by illumination
of upper and lower visual fields. J Biol Rhythms. (1999) 14:122–
5. doi: 10.1177/074873099129000506
25. Glickman G, Hanifin JP, Rollag MD, Wang J, Cooper H, Brainard GC.
Inferior retinal light exposure is more effective than superior retinal
exposure in suppressing melatonin in humans. J Biol Rhythms. (2003) 18:71–
9. doi: 10.1177/0748730402239678
26. Hébert M, Martin SK, Lee C, Eastman CI. The effects of prior light history
on the suppression of melatonin by light in humans. J Pineal Res. (2002)
33:198–203. doi: 10.1034/j.1600-079X.2002.01885.x
27. Smolensky MH, Sackett-Lundeen LL, Portaluppi F. Nocturnal light pollution
and underexposure to daytime sunlight: Complementary mechanisms of
circadian disruption and related diseases. Chronobiol Int. (2015) 32:1029–
48. doi: 10.3109/07420528.2015.1072002
28. Foster RG, Peirson SN, Wulff K, Winnebeck E, Vetter C, Roenneberg
T. Chapter eleven - sleep and circadian rhythm disruption in
social jetlag and mental illness. In: Gillette MU, editor. Progress in
Molecular Biology and Translational Science. Oxford; Amsterdam;
Waltham, MA; San Diego, CA: Academic Press (2013). vol. 119. p.
325–46. doi: 10.1016/B978-0-12-396971-2.00011-7
29. Aschoff J. Exogenous and endogenous components in circadian
rhythms. Cold Spring Harb Symp Quant Biol. (1960) 25:11–
28. doi: 10.1101/SQB.1960.025.01.004
30. de Mairan JJ. Observation botanique. In: Historie de l’Academie Royale des
Sciences. Paris (1729).
31. Peirson SN, Halford S, Foster RG. The evolution of irradiance detection:
melanopsin and the non-visual opsins. Philos Trans R Soc Lond B Biol Sci.
(2009) 364:2849–65. doi: 10.1098/rstb.2009.0050
32. Markwell EL, Feigl B, Zele AJ. Intrinsically photosensitive melanopsin retinal
ganglion cell contributions to the pupillary light reflex and circadian rhythm.
Clin Exp Optom. (2010) 93:137–49. doi: 10.1111/j.1444-0938.2010.00479.x
33. Foster RG, Wulff K. The rhythm of rest and excess. Nat Rev Neurosci. (2005)
6:407–14. doi: 10.1038/nrn1670
34. Hatori M, Panda S. The emerging roles of melanopsin in
behavioral adaptation to light. Trends Mol Med. (2010) 16:435–
46. doi: 10.1016/j.molmed.2010.07.005
35. Khalsa SB, Jewett ME, Cajochen C, Czeisler CA. A phase response curve
to single bright light pulses in human subjects. J Physiol. (2003) 549(Pt.
3):945–52. doi: 10.1113/jphysiol.2003.040477
Frontiers in Psychiatry | www.frontiersin.org 10 May 2021 | Volume 12 | Article 652161
Stefani and Cajochen Practice Review on Existing Lighting Recommendations
36. Minors DS, Waterhouse JM, Wirz-Justice A. A human phase-response curve
to light. Neurosci Lett. (1991) 133:36–40. doi: 10.1016/0304-3940(91)90051-T
37. Badia P, Myers B, Boecker M, Culpepper J, Harsh JR. Bright light effects
on body temperature, alertness, EEG, and behavior. Physiol Behav. (1991)
50:583–8. doi: 10.1016/0031-9384(91)90549-4
38. Cajochen C, Zeitzer JM, Czeisler CA, Dijk DJ. Dose-response
relationship for light intensity and ocular and electroencephalographic
correlates of human alertness. Behav Brain Res. (2000) 115:75–
83. doi: 10.1016/S0166-4328(00)00236-9
39. Campbell SS, Dawson D. Enhancement of nighttime alertness and
performance with bright ambient light. Physiol Behav. (1990) 48:317–
20. doi: 10.1016/0031-9384(90)90320-4
40. Myers BL, Badia P. Immediate effects of different light intensities
on body temperature and alertness. Physiol Behav. (1993) 54:199–
202. doi: 10.1016/0031-9384(93)90067-P
41. Gaggioni G, Maquet P, Schmidt C, Dijk DJ, Vandewalle G. Neuroimaging,
cognition, light and circadian rhythms. Front Syst Neurosci. (2014)
8:126. doi: 10.3389/fnsys.2014.00126
42. LeGates TA, Fernandez DC, Hattar S. Light as a central modulator of
circadian rhythms, sleep and affect. Nat Rev Neurosci. (2014) 15:443–
54. doi: 10.1038/nrn3743
43. Phipps-Nelson J, Redman JR, Dijk DJ, Rajaratnam SM. Daytime
exposure to bright light, as compared to dim light, decreases sleepiness,
and improves psychomotor vigilance performance. Sleep. (2003)
26:695–700. doi: 10.1093/sleep/26.6.695
44. Souman JL, Tinga AM, Te Pas SF, van Ee R, Vlaskamp BNS. Acute alerting
effects of light: a systematic literature review. Behav Brain Res. (2018) 337:228–
39. doi: 10.1016/j.bbr.2017.09.016
45. Gornicka GB. Lighting at work : environmental study of direct effects of lighting
level and spectrum on psychophysiological variables [Phd Thesis 1 (Research
TU/e / Graduation TU/e)]. Eindhoven: Technische Universiteit Eindhoven,
Netherlands (2008).
46. Segal AY, Sletten TL, Flynn-Evans EE, Lockley SW, Rajaratnam SMW.
Daytime exposure to short- and medium-wavelength light did not improve
alertness and neurobehavioral performance. J Biol Rhythms. (2016) 31:470–
82. doi: 10.1177/0748730416659953
47. Chang A-M, Scheer FAJL, Czeisler CA. The human circadian
system adapts to prior photic history. J Physiol. (2011) 589(Pt.
5):1095–102. doi: 10.1113/jphysiol.2010.201194
48. Kozaki T, Kubokawa A, Taketomi R, Hatae K. Effects of day-time exposure to
different light intensities on light-induced melatonin suppression at night. J
Physiol Anthropol. (2015) 34:27. doi: 10.1186/s40101-015-0067-1
49. Smith KA, Schoen MW, Czeisler CA. Adaptation of human pineal melatonin
suppression by recent photic history. J Clin Endocrinol Metab. (2004) 89:3610–
4. doi: 10.1210/jc.2003-032100
50. Avery DH, Eder DN, Bolte MA, Hellekson CJ, Dunner DL, Vitiello MV, et al.
Dawn simulation and bright light in the treatment of SAD: a controlled study.
Biol Psychiatry. (2001) 50:205–16. doi: 10.1016/S0006-3223(01)01200-8
51. Gabel V, Maire M, Reichert CF, Chellappa SL, Schmidt C, Hommes V, et
al. Effects of artificial dawn and morning blue light on daytime cognitive
performance, well-being, cortisol., and melatonin levels. Chronobiol Int.
(2013) 30:988–97. doi: 10.3109/07420528.2013.793196
52. Vandewalle G, Dijk D-J. Neuroimaging the effects of light on non-visual brain
functions. In: Nofzinger E, Thorpy MJ, Maquet P, editors. Neuroimaging of
Sleep and Sleep Disorders. Cambridge: Cambridge University Press (2013). p.
171–8. doi: 10.1017/CBO9781139088268.023
53. Boivin DB, Czeisler CA, Dijk DJ, Duffy JF, Folkard S, Minors DS, et
al. Complex interaction of the sleep-wake cycle and circadian phase
modulates mood in healthy subjects. Arch Gen Psychiatry. (1997) 54:145–
52. doi: 10.1001/archpsyc.1997.01830140055010
54. Golden RN, Gaynes BN, Ekstrom RD, Hamer RM, Jacobsen FM, Suppes T,
et al. The efficacy of light therapy in the treatment of mood disorders: a
review and meta-analysis of the evidence. Am J Psychiatry. (2005) 162:656–
62. doi: 10.1176/appi.ajp.162.4.656
55. Mårtensson B, Pettersson A, Berglund L, Ekselius L. Bright
white light therapy in depression: a critical review of the
evidence. J Affect Disord. (2015) 182:1–7. doi: 10.1016/j.jad.2015.
04.013
56. Stephenson KM, Schroder CM, Bertschy G, Bourgin P. Complex interaction of
circadian and non-circadian effects of light on mood: shedding new light on an
old story. Sleep Med Rev. (2012) 16:445–54. doi: 10.1016/j.smrv.2011.09.002
57. Houser K, Boyce P, Zeitzer J, Herf M. Human-centric lighting: myth, magic or
metaphor? Light Res Tech. (2021) 53:97–118. doi: 10.1177/1477153520958448
58. Federal Office of Public Health FOPH, Artificially generated daylight for
interiors. Künstlich erzeugtes Tageslicht für Innenräume. Bern: FOPH
(2019). Available online at: https://www.bag.admin.ch/bag/de/home/
gesund-leben/umwelt- und-gesundheit/strahlung-radioaktivitaet- schall/
elektromagnetische-felder-emf-uv- laser-licht/licht_beleuchtung.html
(accessed April 29, 2020).
59. Motamedzadeh M, Golmohammadi R, Kazemi R, Heidarimoghadam R.
The effect of blue-enriched white light on cognitive performances and
sleepiness of night-shift workers: a field study. Physiol Behav. (2017) 177:208–
14. doi: 10.1016/j.physbeh.2017.05.008
60. Lok R, Smolders KCHJ, Beersma DGM, de Kort YAW. Light, alertness, and
alerting effects of white light: a literature overview. J Biol Rhythms. (2018)
33:589–601. doi: 10.1177/0748730418796443
61. Pachito DV, Eckeli AL, Desouky AS, Corbett MA, Partonen T,
Rajaratnam SMW, et al. Workplace lighting for improving alertness
and mood in daytime workers. Cochrane Database Syst Rev. (2018)
3:CD012243. doi: 10.1002/14651858.CD012243.pub2
62. Zhang R, Campanella C, Aristizabal S, Jamrozik A, Zhao J, Porter
P, et al. Impacts of dynamic LED lighting on the well-being and
experience of office occupants. Int J Environ Res Public Health. (2020)
17:7217. doi: 10.3390/ijerph17197217
63. Figueiro M, Steverson B, Heerwagen J, Yucel R, Roohan C, Sahin L, et al. Light,
entrainment, and alertness: a case study in offices. Light Res Technol. (2020)
52:736–50. doi: 10.1177/1477153519885157
64. Jensen HI, Markvart J, Holst R, Thomsen TD, Larsen JW, Eg DM, et al. Shift
work and quality of sleep: effect of working in designed dynamic light. Int Arch
Occup Environ Health. (2016) 89:49–61. doi: 10.1007/s00420-015-1051-0
65. Griepentrog JE, Labiner HE, Gunn SR, Rosengart MR. Bright environmental
light improves the sleepiness of nightshift ICU nurses. Crit Care. (2018)
22:295. doi: 10.1186/s13054-018-2233-4
66. de Kort Y, Smolders K. Effects of dynamic lighting on office workers: first
results of a field study with monthly alternating settings. Light Res Technol.
(2010) 42:345–60. doi: 10.1177/1477153510378150
67. Stefani O, Freyburger M, Veitz S, Basishvili T, Meyer M, Weibel J, et al.
Changing color and intensity of LED lighting across the day impacts on
circadian melatonin rhythms and sleep in healthy men. J Pineal Res. (2021)
70:e12714. doi: 10.1111/jpi.12714
68. Schöllhorn I PA, Braun M, Seiler S, Stefani, O. Evaluation eiNes
Alternierenden Beleuchtungskonzepts in Einem Produktionsbetrieb. Arbeit
interdisziplinär Analysieren, bewerten, gestalten : 65 Kongress der Gesellschaft
für Arbeitswissenschaft; 27.Februar - 1.März. (2019). Dresden: Gesellschaft
für Arbeitswissenschaft (GfA) (2019).
69. Moore-Ede M, Heitmann A, Guttkuhn R. Circadian potency spectrum
with extended exposure to polychromatic white LED light under workplace
conditions. J Biol Rhythms. (2020) 35:405–15. doi: 10.1177/0748730420923164
70. Rahman SA, Flynn-Evans EE, Aeschbach D, Brainard GC, Czeisler CA,
Lockley SW. Diurnal spectral sensitivity of the acute alerting effects of light.
Sleep. (2014) 37:271–81. doi: 10.5665/sleep.3396
71. Viola AU, James LM, Schlangen LJ, Dijk DJ. Blue-enriched white light in the
workplace improves self-reported alertness, performance and sleep quality.
Scand J Work Environ Health. (2008) 34:297–306. doi: 10.5271/sjweh.1268
72. Revell VL, Arendt J, Fogg LF, Skene DJ. Alerting effects of light
are sensitive to very short wavelengths. Neurosci Lett. (2006) 399:96–
100. doi: 10.1016/j.neulet.2006.01.032
73. Kelly R. Lighting as an integral part of architecture. Coll Art J. (1952)
12:24–30. doi: 10.2307/773361
74. Hansen EK, Bjørner T, Xylakis E, Pajuste M. An experiment of double
dynamic lighting in an office responding to sky and daylight: perceived effects
on comfort, atmosphere and work engagement. Indoor Built Environ. (2021).
doi: 10.1177/1420326X21991198
75. Hansen E, Pajuste M, Xylakis E. Flow of light: balancing
directionality and cct in the office environment. Leukos.
(2020). doi: 10.1080/15502724.2020.1808014. [Epub ahead of print].
Frontiers in Psychiatry | www.frontiersin.org 11 May 2021 | Volume 12 | Article 652161
Stefani and Cajochen Practice Review on Existing Lighting Recommendations
76. Fleischer S. The Psychological Effect of Changeable Artificial Lighting Situations
on Humans. ETH Zurich Research Collection Zurich (Switzerland). Zurich:
ETH (2001).
77. Houser K, Tiller D, Bernecker C, Mistrick R. The subjective response to
linear fluorescent direct/indirect lighting systems. Light Res Technol. (2002)
34:243–60. doi: 10.1191/1365782802li039oa
78. Hubalek S, Brink M, Schierz C. Office workers’ daily exposure
to light and its influence on sleep quality and mood. Light
Res Technol. (2010) 42:33–50. doi: 10.1177/147715350935
5632
79. Huiberts LM, Smolders KCHJ, de Kort YAW. Non-image
forming effects of illuminance level: exploring parallel effects on
physiological arousal and task performance. Physiol Behav. (2016)
164:129–39. doi: 10.1016/j.physbeh.2016.05.035
80. Smolders KCHJ, de Kort YAW, Cluitmans PJM. A higher illuminance
induces alertness even during office hours: findings on subjective measures,
task performance and heart rate measures. Physiol Behav. (2012) 107:7–
16. doi: 10.1016/j.physbeh.2012.04.028
81. ISO/CIE DIS 11664-2 [PREN ISO/CIE 11664-2]. ICS 17.180.20 Colorimetry -
Part 2: CIE Standard Illuminants. (2020). Available online at: https://www.iso.
org/standard/77215.html (accessed April 29, 2020).
82. /E:2018 CS. CIE system for metrology of optical radiation for
ipRGC-influenced responses to light. Color Res Appl. (2018)
44:316. doi: 10.1002/col.22350
83. Ticleanu. Research Insight Circadian lighting. London: CIBSE and BRE (2020).
84. DGUV. Deutsche Gesetzliche Unfallversicherung e.V. (2018). Available
online at: https://publikationen.dguv.de/widgets/pdf/download/article/3247
(accessed April 29, 2021).
85. DIN SPEC 67600:2013-04. Biologically Effective Illumination - Design
guidelines [Biologisch wirksame Beleuchtung - Planungsempfehlungen]. (2013).
Available online at: https://www.beuth.de/en/technical-rule/din-spec- 67600/
170956045 (accessed April 29, 2020).
86. Underwriters Laboratories (UL). Design Guideline for Promoting Circadian
Entrainment with Light for Day-Active People. (2020). Available online
at: https://www.shopulstandards.com/ProductDetail.aspx?UniqueKey=36592
(accessed April 29, 2021).
87. The WELL Building Standard. Circadian Lighting Design. (2020). Available
online at: https://standard.wellcertified.com/light/circadian-lighting- design
(accessed April 29, 2020).
88. CIE. Position Statement on Non-Visual Effects of Light - Recommending
Proper Light at the Proper Time, 2nd ed. (2019). Available online
at: https://cie.co.at/publications/position-statement- non-visual-effects- light-
recommending-proper-light-proper- time-2nd (accessed April 29, 2021).
89. KAN. KAN-Positionspapier zum Thema künstliche,biologisch wirksame
Beleuchtung und Normung. (2017). Available online at: https://www.kan.de/
fileadmin/Redaktion/Dokumente/Basisdokumente/de/Deu/KAN-Position_
Beleuchtung_2017.pdf (accessed April 29, 2021).
90. IES. PS-12-19: IES Position On UL RP 24480 Regarding Light and Circadian
Entrainment. (2020). Available online at: https://www.ies.org/about-
outreach/position-statements/ps- 12-19-ies- position-on- ul- rp-24480-
regarding-light- and-circadian-entrainment/ (accessed April 29, 2021).
91. CIBSE. Literature Review on Circadian Lighting. (2017). Available
online at: https://www.cibse.org/knowledge/knowledge-items/detail?id=
a0q0O00000CF7o9QAD (accessed April 29, 2021).
92. Brown TM. Melanopic illuminance defines the magnitude of human circadian
light responses under a wide range of conditions. J Pineal Res. (2020)
69:e12655. doi: 10.1111/jpi.12655
93. Nowozin C, Wahnschaffe A, Rodenbeck A, de Zeeuw J, Hädel S, Kozakov R,
et al. Applying melanopic lux to measure biological light effects on melatonin
suppression and subjective sleepiness. Curr Alzheimer Res. (2017) 14:1042–
52. doi: 10.2174/1567205014666170523094526
94. Allen AE, Hazelhoff EM, Martial FP, Cajochen C, Lucas RJ. Exploiting
metamerism to regulate the impact of a visual display on alertness and
melatonin suppression independent of visual appearance. Sleep. (2018)
41:zsy100. doi: 10.1093/sleep/zsy100
95. de Zeeuw J, Papakonstantinou A, Nowozin C, Stotz S, Zaleska M, Hädel
S, et al. Living in biological darkness: objective sleepiness and the pupillary
light responses are affected by different metameric lighting conditions during
daytime. J Biol Rhythms. (2019) 34:410–31. doi: 10.1177/0748730419847845
Conflict of Interest: The authors declare the following potential conflicts of
interest with respect to the research, authorship, and/or publication of this article:
OS is listed as an inventor on the following patents: US8646939B2—Display system
having circadian effect on humans; DE102010047207B4—Projection system and
method for projecting image content; US8994292B2—Adaptive lighting system;
WO2006013041A1—Projection device, and filter thereof; WO2016092112A1—
Method for the selective adjustment of a desired brightness and/or color of a
specific spatial area, and data processing device thereof. OS is a member of the
Daylight Academy. OS has had the following commercial interests in the last
four years (2017–20) related to lighting: Investigator-initiated research grants
from Derungs, Audi, VW, Porsche, Festo, ZDF, Toshiba, and SBB; Speaker fees
for invited seminars from Merck, Fraunhofer, Firalux, and Selux. CC has had
the following commercial interests in the last four years (2017–2020) related
to lighting: honoraria, travel, accommodation and/or meals for invited keynote
lectures, conference presentations or teaching from Toshiba Materials, Velux,
Firalux, Lighting Europe, Electrosuisse, Novartis, Roche, Elite, Servier, and WIR
Bank. CC is a member of the Daylight Academy.
Copyright © 2021 Stefani and Cajochen. This is an open-access article distributed
under the terms of the Creative Commons Attribution License (CC BY). The use,
distribution or reproduction in other forums is permitted, provided the original
author(s) and the copyright owner(s) are credited and that the original publication
in this journal is cited, in accordance with accepted academic practice. No use,
distribution or reproduction is permitted which does not comply with these terms.
Frontiers in Psychiatry | www.frontiersin.org 12 May 2021 | Volume 12 | Article 652161
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We examined whether dynamically changing light across a scheduled 16‐h waking day influences sleepiness, cognitive performance, visual comfort, melatonin secretion, and sleep under controlled laboratory conditions in healthy men. Fourteen participants underwent a 49‐h laboratory protocol in a repeated‐measures study design. They spent the first 5‐h in the evening under standard lighting, followed by an 8‐h nocturnal sleep episode at habitual bedtimes. Thereafter, volunteers either woke up to static light or to a dynamic light that changed spectrum and intensity across the scheduled 16‐h waking day. Following an 8‐h nocturnal sleep episode, the volunteers spent another 11‐h either under static or dynamic light. Static light attenuated the evening rise in melatonin levels more compared to dynamic light as indexed by a significant reduction in the melatonin AUC prior to bedtime during static light only. Participants felt less vigilant in the evening during dynamic light. After dynamic light, sleep latency was significantly shorter in both the baseline and treatment night while sleep structure, sleep quality, cognitive performance and visual comfort did not significantly differ. The study shows that dynamic changes in spectrum and intensity of light promote melatonin secretion and sleep initiation in healthy men.
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This paper discusses the rise of human-centric lighting and its current status in lighting. We summarise the human benefits associated with light and lighting and show that human-centric lighting has sound motivations, despite being tainted by misleading marketing claims. The phrase integrative lighting avoids the hype and encapsulates what lighting aspires to be. Embodied in these concepts are some things old and some things new. The old is twofold. First, without diminishing the value of lighting products, the core ingredient for good human outcomes is good design, driven by a design team. Second, light is still for vision, and lighting for visibility, visual comfort and visual amenity is as important as ever. Complementing the old is new awareness and responsibility for how light and lighting influence non-visual responses in humans. Circadian, neuroendocrine and neurobehavioural responses are important for human health and should be considered on-par with visual responses. This awareness leads toward lighting design solutions with increased contrast between day and night. The parties responsible for addressing non-visual responses to light and lighting are evolving. Architects, lighting professionals, lighting equipment manufacturers, medical professionals, building owners and individuals all have a stake, but who should drive decisions and in what proportion?
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LED *Shared senior authors. light sources have a discontinuous light spectrum with a prominent ‘blue’ peak between 450 and 470 nm that influences non-image forming responses in humans. We tested an LED lighting solution mimicking a daylight spectrum on visual comfort, circadian physiology, daytime alertness, mood, cognitive performance and sleep. Fifteen young males twice spent 49 hours in the laboratory under a conventional-LED and under a daylight-LED condition in a balanced cross over design flanked by a baseline and a post-light exposure night. Despite different light spectra, the photopic lux and the correlated colour temperature of the lighting were the same for both LEDs. The colour rendering index and the melanopic strength were 25.3% and 21%, respectively, higher for the daylight LED than the conventional LED. The volunteers had better visual comfort, felt more alert and happier in the morning and evening under daylight LED than conventional LED, while the diurnal melatonin profile, psychomotor vigilance and working memory performance were not significantly different. Delta EEG activity (0.75–4.5 Hz) was significantly higher after daylight-LED than conventional-LED exposure during the post-light exposure night. We have evidence that a daylight-LED solution has beneficial effects on visual comfort, daytime alertness, mood and sleep intensity in healthy volunteers.