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The effects of red and blue light on alertness and mood at night

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Abstract

This study was designed to explore the roles that long- and short-wavelength lights have on momentary mood and alertness at night. Twenty-two subjects participated in a mixed-design experiment, where we measured the impact of two levels of long- and short-wavelength lights on brain activity and on self-assessments of alertness, sleepiness and mood. Measurements were obtained 60 minutes prior to, during and after light exposure. Results showed that the red and the blue lights increased electroencephalographic beta power (12—30 Hz), reduced sleepiness, and increased positive affect relative to the previous dim-light period indicating that alertness and mood can be affected by light without necessarily stimulating the melatonin pathway. The impact of light was modest, however, compared to the increase in fatigue over the course of the night.
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DOI: 10.1177/1477153509360887
2010 42: 449 originally published online 2 March 2010Lighting Research and Technology
B. Plitnick, MG Figueiro, B. Wood and MS Rea
The effects of red and blue light on alertness and mood at night
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The effects of red and blue light on alertness
and mood at night
B Plitnick RN, MG Figueiro PhD, B Wood and MS Rea PhD FSLL
Lighting Research Center, Rensselaer Polytechnic Institute, 21 Union Street, Troy, NY 12180, USA
Received 23 October 2009; Revised 18 December 2009; Accepted 28 December 2009
This study was designed to explore the roles that long- and short-wavelength
lights have on momentary mood and alertness at night. Twenty-two subjects
participated in a mixed-design experiment, where we measured the impact of
two levels of long- and short-wavelength lights on brain activity and on self-
assessments of alertness, sleepiness and mood. Measurements were obtained
60 minutes prior to, during and after light exposure. Results showed that the red
and the blue lights increased electroencephalographic beta power (12–30 Hz),
reduced sleepiness, and increased positive affect relative to the previous dim-light
period indicating that alertness and mood can be affected by light without
necessarily stimulating the melatonin pathway. The impact of light was modest,
however, compared to the increase in fatigue over the course of the night.
1. Background
Environmental stimuli can affect momentary
mood and alertness. The impact of colour on
momentary mood and alertness is still con-
troversial, at least in part because of the
confusion between three distinct domains,
physical, physiological and psychological.
Strictly speaking colour is not a stimulus,
but rather a psychological result of processing
by neural mechanisms of the spectral irra-
diance distribution (SID) incident on the
retina. Some authors suggest that apparent
colour itself affects momentary mood and
alertness. Other studies have examined the
impact of different SIDs on hormone pro-
duction and electrical brain activity asso-
ciated with momentary mood and alertness
without specific regard to the apparent colour
those stimuli evoke. It is usually very difficult,
however, to determine the underlying
mechanisms contributing to light-induced
changes in momentary mood and alertness
because optical radiation incident on the
retina has multiple effects on brain activity
through parallel neural pathways.
For example, some studies suggest that
colours seen as ‘warm’ (red, orange, yellow)
evoke feelings of arousal while colours seen as
‘cool’ (violet, blue, green) are associated with
calming feelings. The colour red, for example,
has been associated with feelings of danger,
love, rage and excitement as well as negative
feelings such as anxiety, anger and annoy-
ance. It has been suggested that the colour red
increases human receptiveness to external
stimuli and increases excitation, therefore
affecting a person’s emotional and motor
responses.
1,2
Goldstein has argued that red
colours, though arousing, impair perfor-
mance on complex activities in which exact-
ness is required. The colours green and blue
have been associated with feelings of relax-
ation and calmness.
2–6
Elliot et al.
7
performed a series of studies to
investigate the impact of red colour on
performance in achievement contexts, that
is, in situations in which competence is
Address for correspondence: MG Figueiro, Lighting Research
Center, Rensselaer Polytechnic Institute, 21 Union Street,
Troy, NY 12180, USA. E-mail: figuem@rpi.edu
Lighting Res. Technol. 2010; 42: 449–458
ß The Chartered Institution of Building Services Engineers 2010 10.1177/1477153509360887
evaluated and positive and negative outcomes
are possible. They hypothesised that red
colour is associated with danger of failure,
and therefore, there would be an automatic,
unconscious decision to avoid the object,
situation or events. Their findings supported
the hypothesis that perception of red colour
prior to an achievement task impairs perfor-
mance compared to a green and an achro-
matic colour. Similar findings have been
reported by Stone.
8
These results are not
consistent with findings by Hill and Barton,
9
however, who reported that red enhances
performance of athletes who wore red colour.
In general, the studies of colour on emotions
and performance are conflicting. Although
part of the explanation for this lack of
consistency may be due to random, non-
systematic effects of apparent colour on
human physiology and behavior, or to indi-
vidual differences in apparent colour prefer-
ence, or to differences in cultural associations
with apparent colour,
10
some of the uncer-
tainty must arise from a failure to define the
light stimulus independent of the psychologi-
cal response that the stimulus evokes, that is,
its apparent colour. Without a characteri-
sation of the stimulus independent of the
response it evokes, it is not possible to develop
an understanding of the physiological
mechanisms mediating the measured outcome.
Other studies have examined the impact of
SIDs (combinations of both spectrum and
irradiance), not simply apparent colour, on
alertness, sleepiness and performance.
11,12
Of
particular interest for the present study,
several studies have examined the impact of
short-wavelength light on nocturnal alertness
and performance as it might be mediated by
the suprachiasmatic nuclei (SCN), the pace-
maker for regulating circadian rhythms.
Short-wavelength light is an important stim-
ulus to the human circadian system; it is
known to be maximally sensitive to short-
wavelength radiation peaking at about
450 nm. Presumably, it is only a collateral
effect that short-wavelength light is also seen
as blue in these studies. This assumption has
justification because the results of recent
studies using short-wavelength light as a
stimulus for enhancing alertness and perfor-
mance and reducing sleepiness are entirely
consistent with the neurophysiological evi-
dence that neural pathways from the SCN are
important to sleep and alertness.
13
These
converging lines of research add weight to
the inference that the SCN, through retinal
stimulation by short-wavelength light, plays
an important role in human alertness and
probably performance. Very recently, how-
ever, Figueiro and colleagues
14
showed that
the circadian system may not be the only
pathway associated with light-induced alert-
ness at night. They showed that both long-
wavelength, red light and short-wavelength,
blue light of the same corneal illuminance
evoked similar alerting effects (i.e. increased
power in the electroencephalogram (EEG)
beta frequencies and reduced power in the
EEG alpha frequencies).
The goal of this study was to expand on
that work by Figueiro and colleagues by
investigating the impact of long-wavelength,
red and short-wavelength, blue light on mea-
sures of sleepiness and momentary mood
(both measured subjectively). The present
study was expected to serve as a partial
validation of the previous results as well as
to investigate the dissipation of the alerting
effects of light after exposures to blue and
red lights.
2. Methodology
In order to minimise differences in cultural
biases toward colours, 24 native-born subjects
from the United States (19–27 years of age)
were recruited to participate in the study
from an electronic posting at Rensselaer
Polytechnic Institute in Troy, New York.
All subjects were screened for major health
problems and except for women taking birth
450 B Plitnick et al.
Lighting Res. Technol. 2010; 42: 449–458
control pills, subjects reported not taking
any pharmaceuticals or medications. Every
subject completed a Munich Chronotype
Questionnaire (MCTQ) prior to the study;
those who were late or extremely late
chronotypes were excluded from the experi-
ment. This study was approved by the
Rensselaer Polytechnic Institute Institutional
Review Board. Subjects were asked to refrain
from alcohol and caffeine on the days of the
experiment and were asked not to sleep after
awakening for the day. Subjects were also
asked to go to bed no later than 23:00 the night
before the experiment. Of the 24 individuals
recruited for the study, 22 (9 males and
13 females) subjects completed the entire
experiment and their results are reported
here. Two subjects decided to withdraw from
the experiment after the first session.
Four experimental lighting conditions, two
spectra (blue and red) at two levels (10 lx and
40 lx), were delivered to individual subjects
from 0.6 0.6 0.6 m
3
light boxes, each fitted
with arrays of light-emitting diodes (LEDs).
The arrays (ICove, Colour Kinetics) were
located behind the front box apertures to be
outside the subject’s direct view, thereby
creating a uniform, non-glaring distribution
of light within the box. During light expo-
sures, subjects placed their chin on a rest
mounted near the front of a box, ensuring
delivery of the prescribed light exposure. The
spectral emissions of the blue LEDs peaked at
470 nm with a full width at half maximum
(FWHM) of 25 nm. The red LEDs peaked at
630 nm with a FWHM of 25 nm. Before the
experiment, each of the light boxes was
calibrated using a Gigahertz illuminance
photometer to provide 10 lx or 40 lx for both
LED arrays at the plane of the subject’s
corneas when positioned in the chinrest. Two
boxes provided blue light (40.2 mW/cm
2
at
40 lx and 10 mW/cm
2
at 10 lx) and two emitted
red light (18.9 mW/cm
2
at 40 lx and 4.7 mW/cm
2
at 10 lx); light levels were adjusted with an
electronic dimmer without significantly
affecting the spectral irradiance distributions
of the LEDs. Measurements of pupil area
completed after the experiment with a differ-
ent group of subjects (N ¼ 5) were: red at
10 lx, 34 mm
2
; red at 40 lx, 22 mm
2
; blue at
10 lx, 10 mm
2
; blue at 40 lx, 6.5 mm
2
.
The Biosemi ActiveTwo system with active
electrodes was used for EEG recordings. This
system is battery powered, minimising elec-
trical interference from alternating current
(ac) during recording sessions. Electrodes
were placed on subjects’ scalps according to
the International 10-20 system at Oz, Pz, Cz
and Fz.
15
Two additional electrodes serving
as virtual reference electrodes for those
attached to the scalp were attached to the
right and to the left earlobes. Another elec-
trode was placed approximately 5 cm below
the left clavicle to measure an electrocardio-
gram (ECG) signal.
To minimise introducing personal biases
associated with colour into the experiment,
a mixed-design experiment was conducted
whereby half the subjects only saw blue light
and half the subjects only saw red light.
Subjects were given the same instructions
before starting the experiment during which
they were informed that the light colour they
would be seeing (red or blue) was known to
have an alerting effect and that we were
investigating the impact of light level. The
light stimuli were approximately equated in
terms of visual response (i.e. illuminance at
the cornea), not in terms of circadian response
because equal circadian responses by both
spectra would have resulted in exceedingly
bright red-light conditions relative to the
blue-light conditions. Marked disparities in
the apparent brightness of the blue and red
lights would have confounded interpretation
of any subjective responses to the light stimuli
because it would have been impossible to
know whether subjects were responding dif-
ferentially to the apparent brightness or to the
apparent colour. The mixed-design study was
conducted over the course of several weeks
Effects of light on alertness and mood at night 451
Lighting Res. Technol. 2010; 42: 449–458
during February and March 2009. Each
subject saw one light level per night in a
counterbalanced order. Twenty-four subjects
were recruited originally, but since only 11
subjects in each group completed the study
the planned counterbalancing was not com-
plete; six subjects saw the blue and the red
lights at 10 lx first while five subjects saw the
blue and red lights at 40 lx first. All subjects
were asked to arrive at the laboratory at 23:00
to receive instructions and be fitted with
electrodes for EEG/ECG recordings. Groups
of four subjects participated in two sessions
separated by at least 1 week.
Every session began at 00:00 and was
completed at 03:30. Figure 1 shows the
experimental design for one subject. The
start of data collection for each of the other
three subjects in a session was successively
staggered by 5 minutes. The 60-minute light
exposure condition was preceded by a
60-minute period in dim light (51 lx of red
light (
max
¼ 630 nm) at the cornea). Starting
at midnight, subjects remained in the dim
light for 60 minutes. Recordings of the first
subjective assessments (sleepiness, alertness
and momentary mood) in the dim light (D1)
were completed after 15 minutes into the
dim-light period. Self-assessments of sleepi-
ness and alertness were performed using the
Karolinska Sleepiness Scale (KSS)
16
and a
modified Norris mood scale.
12,17
Self-
assessments of momentary mood were
obtained using the Positive and Negative
Assessment Scale (PANAS).
18
KSS is a
9-point standardised sleepiness scale ranging
from ‘extremely alert’ (1) to ‘very sleepy,
fighting sleep’ (9). The modified Norris mood
scale was comprised of 12 items; however, we
only used responses to one, 7-point scale (3,
drowsy, to þ3, alert) in the analyses. The
PANAS scores are each based on 10, 5-point
scales developed to independently measure
positive affect and negative affect.
As shown in Figure 1, the first set of EEG/
ECG measurements, at time (E)R, was
obtained after 60 minutes in dim light,
followed by another set of subjective assess-
ments, at time (S)R, also collected in the dim
light. Subjects were then asked to sit in front
of the (red or blue) illuminated light box for
60 minutes. Subjective assessments (La) were
Preparation
23:00
0:00
D1
(S)R
La
Lb
L
D2a
D2b
D2
D2c
(E)R
1:00
2:00
3:00
EEG/ECG
Self-report
(E)
(S)
First dim (D1)
Light (L)
Second dim (D2)
Reference measurement (R)
Figure 1 Four subjects were scheduled to arrive at the laboratory at 23:00 for preparation for an experimental
session. Data collection for the four subjects in a session was staggered to permit sequential EEG/ECG recordings.
Every session began at 00:00 and data collection was completed for the last of the four subjects at 03:30. Illustrated is
the measurement time sequence for the first subject in a session. During every session, each subject was presented a
high (40 lx) or a low (10 lx) light exposure condition of the same spectrum (blue or red). Subjects within a group
received the same colour of light (red or blue), but half saw the lower and half the higher light level. EEG/ECG
collection times (white bars): The reference measurement (E)R ¼ after 60 minutes in the dim light, L ¼ after 60 minutes
in the light, D2a ¼ 5 minutes after light was turned off; D2b ¼ 25 minutes after light was turned off; D2c ¼ 45 minutes
after light was turned off. Self-report collection times (black bars): D1 ¼ after 15 minutes in the first dim light condition,
(S)R ¼ after approximately 63 minutes in the dim light; La ¼ after 15 minutes in the light condition, Lb ¼ after
approximately 63 minutes in the light condition; D2 ¼ approximately 48 minutes after the light condition was
completed
452 B Plitnick et al.
Lighting Res. Technol. 2010; 42: 449–458
obtained 15 minutes after the red or blue light
was turned on. After 60 minutes of light
exposure, EEG/ECG measurements (L) and
subjective assessment (Lb) were obtained
again. Lights were turned off 20 minutes
after data collection and subjects sat again in
dim light. EEG/ECG measurements (D2a to
D2c) were taken at 5, 25 and 45 minutes after
the experimental treatment light was turned
off. Subjective assessments (D2) were taken
again after the final EEG/ECG measurements
(45 minutes after the light was turned off).
Subjects were asked to perform word searches
or crossword puzzles continuously during the
dim and light periods; these diversions were
placed on clipboards so that subjects could
keep their chins in the chin-rest for the
prescribed exposure duration. For the last
15 minutes prior to EEG/ECG collection,
subjects were asked to complete Sudoku
puzzles. During the last 45-minute of data
collection (after lights were turned off), sub-
jects continuously worked on completing
Sudoku puzzles until each collection time
(5, 25 and 45 minutes after the lights were
turned off).
Near the end of every dim and every light
exposure period, the electrodes affixed to each
subject in a session were, in turn, plugged into
the recording system for EEG and ECG
measurements. Three minutes of continuous
data were collected from each subject. The
subjects were asked to fixate on a specific
marked point on the far side of the light box,
approximately 1 m away. When in the dim
light, subjects were asked to continue to sit in
front of the light box, but the LEDs were not
energised. Subjects were visually monitored
by an experimenter to ensure compliance with
the protocol.
3. Results
3.1 Self-report
Since the order of light level presentations
were counterbalanced within groups across
the two sessions, it was possible to statistically
compare self-reports of alertness, sleepiness,
and affect, both negative and positive, inde-
pendent of the order in which the light levels
were presented to the subjects.
To minimise inherent idiosyncrasies among
the subjects, the difference between responses
given by the subjects during the reference
measurement time (S)R, after 60 minutes in
the dim light, and responses given during the
other conditions (D1, La, Lb and D2) were
used to evaluate the experimental conditions.
Thus, difference scores on the Norris scale,
the KSS scale and the PANAS Positive and
Negative scales were each submitted to a
one-between (colour) by two-within (light
level and time) mixed-design analysis of
variance (ANOVA). Responses to questions
on the Norris scale and on the PANAS
Negative scale exhibited no statistically
significant effects and are not discussed
further.
Difference scores for the KSS and the
PANAS Positive scales were quite similar,
both showing a significant main effect of time
(F
3,60
¼ 18.3, p50.0001 for KSS and
F
3,60
¼ 39.5, p50.0001 for PANAS Positive).
A significant light level time colour inter-
action (F
3,60
¼ 7.3, p ¼ 0.0003) was found for
KSS difference scores and for the PANAS
Positive difference scores (F
3,60
¼ 2.83,
p ¼ 0.046). For the PANAS Positive scale,
student’s two tail t-tests were performed for
the combined light level data and revealed a
significant difference between (S)R-D1 and
(S)R-La (p50.0001), (S)R-D1 and (S)R-Lb
(p50.0001), (S)R-D1 and (S)R-D2 (p5
0.0001). For the KSS, significant differences
between (S)R-D1 and (S)R-La (p50.0001),
(S)R-D1 and (S)R-Lb (p50.0001), (S)R-D1
and (S)R-D2 (p50.0001), (S)R-La and
(S)R-D2 (p ¼ 0.005) and (S)R-La and
(S)R-Lb (p ¼ 0.002) were obtained. To correct
for multiple comparisons, the criterion alpha
level (i.e. p50.05) was adjusted in accordance
with the Bonferroni/Dunn method to
Effects of light on alertness and mood at night 453
Lighting Res. Technol. 2010; 42: 449–458
p50.0083. Figure 2 shows the mean difference
scores standard error of the mean (SEM) for
the KSS difference scores and Figure 3 shows
the mean difference scores and SEM for the
PANAS Positive difference scores. These
figures also show that the mean difference
scores for the KSS and the PANAS Positive
scales are effectively mirror images of one
another. Whether this close correspondence
between the two self-report scales has funda-
mental significance (i.e. increased sleepiness
leads to reduced positive affect but not
increased negative affect) cannot be deter-
mined from this experiment. A future study,
specifically aimed at determining if such
relationships exist, would have to be con-
ducted where attention was given to ‘cali-
brating the self-report data with objective
measures of sleepiness and mood.
19
Moreover, both red and blue lights had
similar impact on KSS and PANAS Positive
difference score, suggesting that there was no
differential impact of light spectra on both
measures. As the significant three-way
interactions suggests, however, there were
differential effects for the blue and red lights
at 10 lx and 40 lx with the KSS and PANAS
Positive difference scores or, perhaps, that
subjects within the two groups associated
with the blue and red lights responded
differently on these self-report scales. The
blue light at 40 lx had, as might be expected,
a more positive effect on sleepiness and
momentary mood than did the blue light at
10 lx. A different pattern resulted from the red
light exposures. Red light at 10 lx had a
greater impact on reducing sleepiness and
improving positive affect than did the red
light at 40 lx. Another experiment would need
to be conducted to determine if this effect is
reliable and, if so, what was its basis.
There was no evidence for persistence in
these effects in the following dim-light periods
using any of the self-report measures. Rather,
there was only evidence for a steady increase
in subjective sleepiness and reduction in
positive affect over the course of the experi-
ment using these scales.
PANAS+
Mean difference score
6
4
2
0
–2
–4
–6
–8
R-D1 R-La
Self-report (S)
R-Lb R-D2
Figure 3 Mean difference scores SEM for responses to
the PANAS Positive (þ) scale over the course of the night.
Data for each subject are based upon the change in
response at the reference time (S)R, that is, at the end of
first dim-light period, to those at each of the other
sampling periods, D1, La, Lb and D2
1.50
KSS
Mean difference score
1.25
1.00
0.75
0.50
0.25
0
0.25
0.50
0.75
–1.00
–1.25
R-D1 R-La
Self-report (S)
R-Lb R-D2
Figure 2 Mean difference scores SEM for responses to
the KSS scale over the course of the night. Data for each
subject are based upon the change in response at the
reference time (S)R, that is, at the end of first dim-light
period, to those at each of the other sampling periods D1,
La, Lb and D2
454 B Plitnick et al.
Lighting Res. Technol. 2010; 42: 449–458
3.2 EEG
The EEG signals were sampled at
16 384 Hz and then low-pass filtered and
downsampled to 2048 Hz for electronic stor-
age by the Biosemi system. All subsequent
EEG data processing and analyses were
performed with Matlab, version R2008a by
The Mathworks. The signals recorded from
the two reference channels were averaged and
these values were subtracted from those
obtained from all of the other channels. The
direct current (dc) offset of each channel was
eliminated by subtracting the mean value of
each channel from itself. A low-pass finite
impulse response (FIR) filter (f
3dB
¼ 50 Hz)
was applied and the data were downsampled
to 512 Hz. Then a high-pass, third-order
Butterworth filter (f
3dB
¼ 4 Hz) was applied
to the downsampled signals from each chan-
nel to eliminate slow trending in the data.
Another program divided the filtered data
into 5-second epochs. Eye blink artefacts were
eliminated by removing epochs from all
channels where voltage fluctuations of any
epoch exceeded 100 mV. A Blackman
window followed by a fast Fourier transform
(FFT) was then applied to the data segments.
This process yielded spectral power distribu-
tions from 1 to 50 Hz. The power spectra for
each 1-minute segment were then combined to
give an average spectral power distribution
for each trial. The relative power levels for the
3 minutes in the alpha (8–12 Hz), alpha–theta
(5–9 Hz), theta (5–7 Hz), beta (12–30 Hz) and
gamma (30–50 Hz) ranges were calculated as
a percentage of overall power from 1 to
50 Hz.
The ECG data were digitally processed the
same way as the EEG data up to the high-
pass filtering. For the ECG analysis, the
high-pass filtering 3dB cut-off was lowered
to 0.2 Hz. Heart rates corresponding to the
filtered ECG data were determined by two
methods: (1) by taking the FFT of the ECG,
whereby the frequency having the peak power
within the range from 40 to 120 beats/minute
is the heart rate, and (2) determining the
elapsed time between the QRS complexes (the
successive peaks, Q then R then S, in the ECG
signal) of the ECG.
20
The QRS complex
represents ventricular depolarisation. It is
called a complex because there are three
different waves in it (Q-wave, R-wave and
S-wave). The QRS complexes were located by
the first derivative of the ECG falling below a
negative threshold value after individual
normalisation of first derivative of the ECG.
As with the self-report data, the differences
between relative power in the alpha and the
beta frequency bands obtained from each
subject after 60 minutes in the dim light (E)R
and during the subsequent conditions
(L, D2a, D2b and D2c) were used to evaluate
the experimental conditions. Power differ-
ences in the alpha and in the beta frequency
bands were both submitted to a one-between
(colour) by three-within (light level, time of
measurement and channel) mixed-design
ANOVA. A statistical significant main effect
of time of measurement was found for the
power differences in the beta frequencies
(F
3,60
¼ 4.0, p ¼ 0.012). Figure 4 shows that
unlike the positive change in the relative
power in the beta frequencies for subsequent
dim-light conditions, the change in relative
power in the beta frequencies was negative for
the light exposure condition, (E)R-L, indicat-
ing higher relative beta power during the light
exposure period than during the preceding
dim-light period. Post hoc two-tail student’s
t-tests on the beta power differences were
performed and revealed a significant difference
between (E)R-L and (E)R-D2a (p ¼ 0.046),
(E)R-L and (E)R-D2b (p ¼ 0.015) and (E)R-L
and (E)R-D2c (p ¼ 0.0016). It is perhaps
worth noting too, that the gradual increase
in the difference between beta power after the
light exposure (L) and after dim-light expo-
sures (D2a, D2b and D2c) suggests a gradual
dissipation of the light’s alerting effect; the
difference after 5 minutes is smaller than after
45 minutes in dim light. Consistent with the
Effects of light on alertness and mood at night 455
Lighting Res. Technol. 2010; 42: 449–458
self-report data, beta power after exposure to
both red and blue lights increased relative to
beta power after the previous dim-light
period, suggesting that both light spectra
impacted brain activities.
These results are consistent with the
hypothesis that mental activity increased in
the light exposure conditions relative to the
dim conditions. Consistent with the results for
changes in beta power, although they were
not statistically significant, the change in
relative power in the alpha frequency band
was greatest for the subsequent light exposure
period, with progressively smaller differences
associated with each successive dim period.
3.3 Heart rate
The FFT and QRS heart rate data were
treated in the same way as the self-report and
EEG data. The differences between heart
rates measured after 60 minutes in the dim
period, (E)R, and during the subsequent
conditions (L, D2a, D2b and D2c) were
submitted to a one-between (colour) by
two-within (light level and time) mixed-design
ANOVA. No significant effects were obtained
using the FFT method but there was a
significant colour by time of measurement
interaction (F
3,60
¼ 3.2, p ¼ 0.03) using the
QRS method. The difference between (E)R
and L for blue light conditions was signifi-
cantly greater (p ¼ 0.02) than for the red light
conditions. This interaction is difficult to
interpret because the effect may be due either
to differential effects of the two light spectra
on heart rate or to differential responses to
light by the two groups of subjects who each
saw different spectra. A different experimental
design would be needed to resolve the
question.
4. Discussion
The present study investigated the impact of
long-wavelength, red and short-wavelength,
blue light on subjective and objective mea-
surements of alertness, sleepiness and
momentary mood. Although the mixed-
design experiment did not allow for a precise
comparison of effects of the two light spectra,
both the red and the blue lights reduced
self-reports of sleepiness (decreased KSS) and
improved self-reports of momentary mood
(increased positive affect on the PANAS
scale). Objectively, the light stimuli increased
alertness relative to the dim condition as
measured by the power in the beta frequency
range of the EEG recordings. There was also
a gradual reduction in beta power after the
blue and red lights were turned off suggesting
a small persistence effect of light on alertness.
Further studies are recommended to investi-
gate the persistence of light’s effect after the
stimulus is removed.
In general, these results are consistent with
the literature in that light can have an
influence on people at night as measured by
increased brain activity, reduced self-reports
of sleepiness, and improved self-reports of
momentary mood. It is still not clear, how-
ever, how light stimuli affect these outcomes.
Beta
Mean difference power
0.02
0.015
0.015
0.01
0.01
0.005
0.005
0
R-L R-D2a
EEG/ECG (E)
R-D2b R-D2c
Figure 4 Mean difference and SEM for beta power. Data
for each subject are based upon the change in relative
power in the beta frequency range from the end of the
first dim-light period, (E)R, to those at each of the other
sampling periods, L, D2a, D2b and D2c
456 B Plitnick et al.
Lighting Res. Technol. 2010; 42: 449–458
It seems well established now that light, acting
through the SCN, can affect sleep and alert-
ness at night. Consistent with previous
studies,
14,21,22
however, it would also seem
that more than one mechanism, not just the
melatonin pathway, must be involved because
both long-wavelength, red and short-
wavelength, blue lights led to similar effects
in the present experiment. Finally, it should
be pointed out that developing an under-
standing of these mechanisms will not be
simple because the effects of light on alertness
and momentary mood are relatively small
compared to the overall increase in fatigue
experienced by subjects throughout the long
periods of wakefulness required during the
study nights.
Acknowledgements
The Boeing Company provided financial sup-
port for the study. The authors would like to
acknowledge Andrew Bierman, Nick Meyer,
Karen Kubarek, Dan Wang, Ranjith Kartha
and Bonnie Westlake of the LRC for their
contributions to the project. The authors
would also like to thank Melanie Kimsey-Lin
of Boeing for her support during the project.
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