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Bright Light Effects on Mental Fatigue
K. C. H. J. Smolders, & Y. A. W. de Kort
Eindhoven University of Technology, Eindhoven, the Netherlands
During workdays, we use and deplete
mental resources. Accumulation of effort
spent throughout the workday might result in
increased feelings of sleepiness, lack of
energy, psychological stress and decrements
in performance. Bright light, on the other
hand, has been shown to positively impact
alertness, vitality and performance and may
thus counteract fatigue by helping recover
decreased mental resources. In the present
study, we investigate whether lighting (i.e.,
illuminance) particularly benefits office
employees who suffer from resource
Research has shown that light is important
for our wellbeing, health and performance.
Light can, for instance, have both direct and
phase shifting effects on people’s circadian
rhythm (see e.g. Dijk & Archer, 2009). In
addition, studies have shown that exposure to
higher illuminance levels can result in
increased feelings of alertness and better
performance at night (Cajochen, Zeitzer,
Czeisler & Dijk, 2000; Campbell & Dawson,
1990). Moreover, light shows similar
beneficial effects during daytime if
individuals have first experienced substantial
light or sleep deprivation (Phipps-Nelson,
Redman, Dijk & Rajaratman, 2003; Rüger,
Gordijn, de Vries & Beersma, 2006). A
recent study by Smolders, de Kort and
Cluitmans (2012a, 2012b) revealed beneficial
effects of bright light exposure also during
daytime under regular circumstances. This
study showed that even in the absence of
sleep or light-deprivation, higher illuminance
at eye level can improve employees’
alertness, vitality and objective cognitive task
performance, and influence physiological
arousal measured with heart rate, heart rate
variability (HRV) and EEG. In the latter
study, effects on subjective alertness and
vitality, and physiological arousal were
immediate and consistent during the hour of
bright light exposure. In contrast, the effects
on performance and HRV were dependent on
duration of exposure: These effects were
most pronounced towards the end of the light
exposure. A potential explanation for the
delayed effect of bright light on cognitive
performance is that more intense light
improves cognitive performance mainly
when participants suffer from mental fatigue.
This is consistent with research showing that
light exposure at night or among sleep-
deprived participants can improve reaction
times immediately (Phipps-Nelson et al.,
2003; Lockley, et al., 2006). Furthermore, a
lab study showed that participants who did
not respond to exposure to a higher
illuminance already had faster response times
than participants who did, suggesting that
they did not benefit from bright light because
they already were very alert (Vandewalle et
al., 2006). In the current study, we
investigate whether daytime exposure to a
higher illuminance level has an alerting and
vitalizing effect mainly when a person is
suffering from mental fatigue and resource
In the current study, a 2x2 within-subjects
design (N = 28; 106 sessions
) was applied to
explore effects of two illuminance levels
(200 vs. 1000 lx at eye level, 4000 K) after
mental fatigue induction (fatigue vs. control).
Participants came to the lab on four separate
visits during the same timeslot in the
morning (9:00am, 10:20am or 11:45am) or in
the afternoon (1:15pm, 2:45 or 4.15pm). The
conditions were counterbalanced across
Four participants were not able to participate in
the fourth session and in two sessions the lighting did
not work properly.
Before the start of each session,
participants applied electrodes for heart rate,
skin conductance and temperature measures
according to the instructions given by the
experimenter. Every session started with a 7-
minute baseline phase consisting of a 1-
minute rest period, performance tasks and a
short questionnaire. Baseline performance
was measured using three different tasks: A
3-minute auditory Psychomotor Vigilance
task (PVT), a 1-minute auditory Go-NoGo
task and a 1-minute 2-back task. After the
baseline measurements, the mental fatigue
vs. control manipulation started, which took
about 29 minutes. After this, participants
completed a short questionnaire. During the
baseline measurements and fatigue induction,
participants experienced 200 lx and 4000K at
the work plane.
After the fatigue vs. control manipulation,
participants were exposed to 200 lx or 1000
lx (at the eye) for 30 minutes. During this
light exposure, subjective and objective
measures were administered in two repeated
measurement blocks. Each block started with
a 1-minute rest period. Subsequently,
performance was measured with a 5-minute
auditory PVT, a 3-minute auditory Go-NoGo
task and a 3-minute 2-back task. At the end
of each block, participants completed a short
questionnaire (see Figure 1).
At the end of each session, participants
completed questions concerning subjective
self-control, their evaluation of the lighting
and the environment, time of going to sleep
the night before, time of awakening and time
spent outside. In addition, at the end of the
last sessions, questions concerning person
characteristics, such as light sensitivity,
chronotype and trait vitality, were
administered. Every session lasted 75
minutes and the participants received a
compensation of 12,50 Euros per session.
Mental fatigue induction
Mental fatigue was induced with two
demanding tasks: a 9-minute Multi-Attribute
task battery (MATB) and a 20-minute
modified Stroop task. The MATB is a multi-
task using a flight simulation in which the
participant keeps track of multiple parallel
processes (maintaining the volume in two
fuel tanks, repairing the fuel system when
broken, monitoring the aircraft, tracking the
aircraft with a joystick, and adjusting the
communication channel when needed).
Participants were instructed to keep track of
all parts and perform the tasks as well as
After the MATB, participants engaged in
a modified Stroop colour-naming task. Each
word was presented for 1 second with a 2.2-
second interval between the words. For each
word, participants had to indicate the colour
of the ink by pressing the corresponding key
on the keyboard, except when the word was
presented in red in one version or in yellow
in another version. In these cases,
participants had to name the text instead of
the ink.
In the control condition, participants
watched a 9-minute nature movie and then
read magazines for 17 minutes. At the end of
the control condition, participants engaged in
a 3-minute Stroop task with congruent trials.
Subjective sleepiness was measured with
the Karolinska Sleepiness scale (KSS;
Åkerstedt & Gillberg, 1990). Vitality and
tension were assessed with six items selected
from the Activation-Deactivation checklist
(Thayer, 1989). In addition, two items
assessing positive and negative affect (happy
and sad) were administered in this
questionnaire. Subjective state self-control
was assessed at the end of each session using
six items selected from the State Self-Control
Capacity Scale (Ciarocco, Twenge, Muraven
& Tice, under review).
Fig. 1: Schematic overview procedure.
Movie Reading magazi nes
7 min 9 min 17 min 3 min 1 13 min 17 min
Modified Stroop task
Fatigue induction vs. control
200 lx at desk (4000 K) 200 lx vs. 1000 lx at eye level (4000 K)
Lighting condition
Measurement block 1 Measurement block 2
Three tasks were employed to assess
cognitive performance. An auditory PVT
assessed sustained attention. An auditory Go-
NoGo task measured executive functioning
and inhibition. In addition, a 2-back task was
administered as a measure for working
memory and executive functioning. During
this task, characters were presented on the
screen after each other and participants had
to press the spacebar as fast as possible if the
character presented was the same as two
characters before. Each character was
presented for 200 ms with an interval of 800
ms between two characters.
Physiological arousal was investigated
using heart rate, skin conductance and
temperature measures. These variables were
measured continuously during the
experiment using TMSi software.
Statistical analysis
Linear Mixed Model (LMM) analyses
were performed with Lighting condition,
Fatigue induction (fatigue vs. control) and
Measurement block as predictors (separate
analyses for each dependent variable). In
these analyses, Participant was added as
random variable to group the data per
participant, i.e. to indicate that each
participant was measured multiple times. To
control for differences at baseline, the
baseline measurement was added as covariate
in the analyses. In addition, person
characteristics were added as covariates to
control for these variables.
In this section, we will report the first
results of the effects of Lighting condition
and Fatigue induction on subjective measures
of sleepiness, vitality, mood and self-control,
and PVT performance.
Effects of lighting and fatigue induction on
subjective measures
A manipulation check of the fatigue
induction revealed that participants felt
sleepier and less energetic immediately after
the fatigue induction compared to the control
condition (all p < .01).
Results during the light exposure revealed
a main effect of Fatigue induction on
subjective sleepiness and vitality suggesting
that the effect of the manipulation on these
variables lasted also during the light
exposure (both p < .01).
Lighting condition had a main effect on
subjective feelings of sleepiness (p = .02) and
vitality (p < .01): Participants reported lower
feelings of sleepiness and more vitality in the
1000 lx compared to the 200 lx condition.
These results replicate the earlier findings by
Smolders et al. (2012). For the current
research question, we were mainly interested
in the interaction between Lighting condition
and Fatigue induction. This interaction
approached significance for the KSS (p =
.06) showing an effect of Lighting condition
only after the fatigue induction (p < .01), but
not after the control condition (p = .78). The
interaction effect between Lighting condition
and Fatigue induction on vitality was not
significant (p = .59) suggesting that the effect
on feelings of energy was not moderated by
mental fatigue. Measurement block had no
effects on the subjective measures (all p >
.10) suggesting that the effects were
immediate and consistent throughout the
Effects of lighting and fatigue induction on
Results of the PVT revealed that
participants had slower reaction times after
the fatigue induction compared to the control
condition (p < .01). Lighting condition had
no significant main effect on the mean
reaction times of the PVT (p = .61).
Measurement block had a significant main
effect on the mean reaction times with slower
responses in Block Two than Block One (p <
.01). There was also a marginally significant
interaction between Lighting condition and
Measurement block (p = .06) suggesting a
trend for slower reaction times in the 200 lx
than in the 1000 lx condition only in Block
Two (p = .06) and not in Block 1 (p = .33).
The interaction between Lighting condition
and Fatigue induction was, however, not
significant (p = .84).
At the conference, the results of the Go-
NoGo task and 2-back tasks will also be
The results of the current study suggest
that exposure to bright light has an
immediate and positive effect on subjective
feelings of alertness and vitality. The effect
on sleepiness was moderated by the fatigue
induction, suggesting that 1000 lx (at eye
level) had an effect on sleepiness only when
the participants suffered from mental fatigue.
Although we expected a comparable
direct effect of bright light on performance
on the PVT when participants felt mentally
fatigued, the effect of bright light on
sustained attention seemed to only depend on
duration of exposure. In line with results by
Smolders et al. (2012), participants
performed better on the PVT towards the end
of the bright light exposure. Current study
suggests a delayed effect of bright light on
mental performance regardless of the mental
fatigue status (fatigued vs. relaxed) prior to
the light exposure.
Results of the other performance tasks and
physiological measures will provide
additional insights in the effect of bright light
after mental fatigue during daytime and
normal office hours.
We would like to thank the Intelligent
Lighting Institute Eindhoven for their
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... This is not irrelevant because, during workdays, since we use and reduce greatly our mental resources, it is easy to increase the feeling of sleepiness, lack of energy, psychological stress and in consequence a decrease in performance. Bright light (high illuminance levels) can influence alertness in a positive way, increasing human performance and may counteract fatigue by helping to recover decreased mental resources [18]. Intense light improves cognitive performance, because there are fewer feelings of sleepiness with 1.000 lx when compared with 200 lx [18]. ...
... Bright light (high illuminance levels) can influence alertness in a positive way, increasing human performance and may counteract fatigue by helping to recover decreased mental resources [18]. Intense light improves cognitive performance, because there are fewer feelings of sleepiness with 1.000 lx when compared with 200 lx [18]. Since intermittent pulses have a greater resetting efficacy, it is a way to delay the phase of the circadian rhythm. ...
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