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Light effects on mental wellbeing and performance during office hours
Karin Smolders
Human Technology Interaction, Eindhoven University of Technology Eindhoven, the
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Netherlands, [email protected]
Title of Ph.D. program: Natural Lighting Environments – User performance and wellbeing
Supervisors: Yvonne de Kort and Cees Midden
State of development of thesis: preliminary analyses and results
Cultural and environmental context in which your research is being conducted:
Environmental psychology, Chronobiology, Occupational psychology and Lighting
research
1. Introduction
1.1 Research problem and its novelty
During workdays, we use and deplete mental resources (Kaplan & Kaplan, 1989; Hartig
& Staats, 2003). Accumulation of effort spent throughout the workday might results in
increased feelings of sleepiness, lack of energy, psychological stress and decrements in
performance (e.g. Kaplan & Berman, 2008; Hagger, Wood, Stiff & Chatzisarantis, 2010).
In this study, we investigate whether office lighting, and in particular the amount of light,
i.e. illuminance, can improve office employees’ alertness, vitality and performance during
daytime. More specifically, we focus on whether exposure to bright light can improve
people’s feelings of alertness, vitality and mood, restore their attention and replenish
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their resources, and enhance cognitive performance during daytime.
Chronobiology research has shown that light is important for our mental wellbeing,
health and performance. Research has, for instance, established that light can have both
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direct and phase shifting effects on people’s circadian rhythm (see e.g. Dijk & Archer,
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2009). In addition to these physiological effects, studies have shown that exposure to
higher illuminance levels can result in feelings of increased alertness and better
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performance (Cajochen, Zeitzer, Czeisler & Dijk, 2000; Campbell & Dawson, 1990;
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Phipps-Nelson, Redman, Dijk & Rajaratman, 2003; Rüger, Gordijn, de Vries & Beersma,
2006). These studies, however, were mainly performed during night time or under
unnatural conditions (e.g. sleep and/or light deprivation). In contrast, current research
assesses the effects of light on mental wellbeing and performance under usual office
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work conditions.
1.2 Research aims, questions, hypotheses
In the current experiment, we investigated whether exposure to a higher
illuminance level has an effect on alertness and performance during daytime. We
monitored the occurrence and development of such effects during an hour of light
exposure under natural conditions (i.e., no sleep deprivation or pre-treatment under
extremely low light settings). As levels of alertness, performance and physiological
arousal seem to show diurnal variations (e.g. Blatter & Cajochen, 2007; Valdez, Reilly &
Waterhouse, 2008), we distinguished between morning and afternoon exposure. We
expected to see positive effects of illuminance on subjective measures of alertness and
vitality in line with Rüger et al. (2006) and Phipps-Nelson et al. (2003) who reported such
effects also during daytime, although for more extreme light manipulations, after hours of
relative darkness and/or sleep deprivation. We expected that these effects would be
immediate, i.e. appear with the onset of the light and irrespective of time of day. In
addition, we expected to see no or subtle effects on performance, as such effects have
only been reported after light deprivation (Rüger et al., 2005a) or after sleep deprivation
(Phipps-Nelson et al., 2003). In addition, the effects of illuminance on subjective
evaluations of the lighting and experience of the space were investigated as these
subjective appraisals and associations might mediate subjective alerting and vitalizing
effects of light as suggested by Veitch and colleagues (2008)
2. Method
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The effect of the illuminance level on subjective measures of alertness and vitality, task
performance and physiological arousal was assessed. In addition, effects of bright light
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exposure on subjective appraisals of the lighting and environment were investigated.
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2.1 Research strategy, samples and methodology
In the current experiment, a mixed-group design (N=32) was applied to explore effects of
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two illuminance levels (200 vs. 1000 lux at eye level) for morning versus afternoon
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exposure. On two to four separate visits to the lab, participants first experienced
baseline settings (200 lux at work plane) for 30 minutes. They were then exposed to the
experimental illuminance levels with a wall-mounted luminaire for one hour. Self-report,
performance tasks and physiological measures (i.e. EEG, heart rate and skin
conductance) were applied to assess alertness, vitality, performance and arousal. In
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addition, questions concerning the evaluation of the lighting and environment were
administered at the end of each session.
2.2 Data collection and data analysis
The experiment consisted of five measurement blocks with each part containing two
performance tests and a (short) questionnaire. The first measurement block took place
during the baseline phase; the remaining four measurement blocks comprised the
experimental phase. Each block started with an EEG protocol to measure brain activity
with eyes open and eyes closed. Subsequently, performance was measured with a 5minute auditory Psychomotor Vigilance Test (PVT). In the baseline and Block Four, this
task was followed by the Necker Cube Pattern Control task1 and a questionnaire. In
Blocks One, Two and Three, participants performed a Letter Digit Substitution Test
(LDST) after the PVT and filled in a short version of the questionnaire.
Linear Mixed Model (LMM) analyses were performed to investigate the effect of
Illuminance level, Time of day and Measurement block on subjective measures of
alertness and mood, cognitive performance, physiological measures and evaluation of
the lighting and environment (separate LMM analyses were run for each dependent
variable). In these analyses, participant was added as random variable to group the data
per participant, i.e. to indicate that the same participant was measured multiple times.
Person characteristics, such as light sensitivity and chronotype, were added as
covariates to control for these variables.
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3. Results and discussion
Results on both subjective measures and task performance showed effects of
illuminance on alertness, vitality and attention. Participants felt less sleepy and more
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energetic in the high versus the low lighting condition (both p < .01). In addition,
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participants had shorter reaction times on a sustained attention task in the 1000 lux
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More than one-third of the participants did not appear to comply with the instructions as they indicated
more reversals in part two than in part one. Therefore, we will not report on these data.
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condition (p < .01).2 Results on the heart rate measures suggest that illuminance level
also has an influence on autonomic nervous activity. Heart rate increased compared to
baseline in the 1000 lux condition, while it decreased in the 200 lux condition (p < .01).
In addition, a higher illuminance increased the LF/HF power ratio compared to baseline
suggesting a relative increase in sympathetic activity in the 1000 lux condition (p < .05).
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Subjective evaluations of the lighting revealed that the participants rated the two lighting
conditions differently. They experienced the 1000 lux condition as more bright and
activating, although less pleasant, but judged both illuminance conditions equally
adequate for working (all p < .01). Nevertheless, they felt more alert and vital in the 1000
lux condition compared to the 200 lux condition.
Effects of illuminance on the subjective measures of alertness and vitality were
not dependent on time of day or duration of exposure. Performance effects, on the other
hand, were most pronounced towards the end of the experiment. Reaction times on the
PVT were shorter in the 1000 lux condition compared to the 200 lux condition in Block
Three and Four. Although performance on the LDST started off worse in the 1000 lux,
participants performed better during the later part of the experiment than in the 200 lux
condition. In addition, the effect of illuminance on reaction times was moderated by time
of day: the effect was most pronounced in the morning sessions. The effect on heart rate
was independent of time of day or duration of exposure, while the effect on the LF/HF
power ratio was most pronounced at the end of the light exposure.
These results show that even under natural conditions, i.e. non sleep or light
deprived conditions, a higher illuminance at eye level can improve not only employees’
subjective feelings of alertness and vitality, but also objectively measured performance.
The results of the performance measures suggest that illuminance can have an
activating effect, i.e. improve performance and result in faster responses and higher
accuracy on relatively simple cognitive tasks. These performance effects emerged
particularly in the morning and after prolonged exposure (>30 minutes). In addition, the
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results on heart rate (variability) showed that exposure to a higher illuminance can also
increase physiological arousal.
The light exposure in the current study was relatively brief (i.e., one hour). As
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exposure to a higher illuminance level in workplaces would be less energy efficient,
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future research should investigate the effects of prolonged exposure periods and/or the
effects of exposure to bright light only when activation is needed (e.g. when a person is
Lighting condition did not have a significant main effect on the LDST measures (all p > .10)
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suffering from mental fatigue). This would also render a better understanding of the
underlying mechanisms of the alerting and vitalizing effect of bright light.
The current study suggests that levels of 1000 lux on the eye (as compared to
200 lux, under normally entrained conditions) are sufficient to (temporarily) help
overcome tiredness and decreased vitality, and improve performance on tests of
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vigilance and sustained attention during daytime. These results complement earlier
studies to the positive effects of illuminance on alertness and performance at night and
after sleep deprivation.
4. Continuation of the project
A potential explanation for the delayed effect of illuminance on cognitive performance is
that more intense light improves cognitive performance mainly when participants suffer
from mental fatigue. In a following-up study, we investigate whether the effects of bright
light exposure mainly occur when activation is needed, i.e. whether a higher illuminance
level has mainly an alerting and vitalizing effect when a person is suffering from mental
fatigue and resource depletion. In this experiment, mental fatigue (fatigue vs. control) is
induced after which participants are exposed to either a high or a low illuminance level
(200 vs. 1000 lux at eye level).
In addition, a study was designed as a first step to get insights in the underlying
mechanisms of the alerting and vitalizing effect of bright light during daytime as we don’t
know whether the effect is due to biological processes (photoreceptors activating the
central nervous system) or via psychological processes (e.g. people believe that they will
perform better in more intense light). In this experiment, participants are exposed to
either a constant bright light scenario (i.e. 1000 lux at eye level) or a dynamic lighting
scenario (i.e. an increasing or decreasing illuminance level). All three conditions had the
same light dosage as we induced a brief noticeable change in intensity (i.e. increase or
decrease) at the beginning of each measurement block followed by a gradual and
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unnoticeable change in intensity (i.e. decrease vs. increase, respectively) in the dynamic
lighting scenarios. We expect that if beliefs play an important role in the beneficial effect
of light, increasing the lighting would have a stronger effect on alertness and
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performance than exposure to the constant lighting scenario or decreasing lighting
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scenario. If the amount of photons is the most important, we do not expect to find
differences between the three lighting scenarios as participants in all three conditions
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experience the same light dosage.
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Complementing these studies, a series of experiments is performed to
investigate whether lighting preferences reflect the beneficial effects of bright light on
alertness, vitality and performance. In this study, we explored whether time of day,
daylight contribution, alertness and mood have an influence on light preference. We
hypothesized that people would prefer more intense light when they felt less alert. In
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these experiments, respondents participated in two to four separate visits. After baseline
measurements of alertness and performance, the illuminance level was dimmed and
participants were asked to select the lighting level they felt would be optimal for
performance on a subsequent attention task. The first results of these studies suggest
that subjective alertness and vitality influence people’s light preferences: when
participants suffered from sleepiness or a lack of energy, they preferred a higher
illuminance level than when they felt more alert and energetic. Participants preferred a
higher illuminance than current standards for office environments, especially when
participants felt sleepy and less vital. Data analyses of the second experiment (with
daylight) are still ongoing and will provide insights into whether daylight exposure also
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plays an important role in light preferences throughout the day.
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