The effects of personalized light schedules during daytime hours on energy level and sleep quality of office workers

Light is not only necessary for vision, but it also has non-image forming
effects on mood, alertness and performance. Most studies tested these effects
during the night or the early-morning/late-evening hours. Recently studies
showed that also daytime light is able to increase alertness and improve
performance. The goal of the current study is to test the importance of timing
of light exposure during office hours, investigate individual variation in
responses, and increase knowledge on the mechanism through which light is
effective in improving daytime performance.

Objective: (1) Is an increase in blue light exposure during daytime office
hours effective in increasing subjective alertness and performance levels? (2)
Is the timing of light exposure important for this effect and is it different
for different chronotypes? (3) Is this increase of alertness only an acute
effect during exposure or is there also a long-term effect through a better
quality of sleep.

In a cross-over design 45 subjects will participate in a control condition and
three experiment light conditions. Each condition will last 2 weeks. In the
three experimental conditions light intensity of, mainly blue, light will be
increased during 3 hours each day, spread over the day, during at least 4
working days per week.

Each condition lasts 2 weeks: (1) normal office lighting (full spectrum light
vertical intensity ~ max 200 lux; (2) extra blue light during first 3 hours ..;
(3) extra blue light during middle 3 hours ..; (4) extra blue light during last
3 hours of daytime working period.

There are no risks involved when participating in this project. Light intensity
is much lower compared to the regular exposure to natural light or light of
light therapy devices. The light intensity is only increased relative to
regular office lighting. The burden for the subjects is relatively low.
Subjects perform the measurements mainly at home or during office hours. Due to
this, employers have to agree with participations. Wearing an activity meter at
the wrist is not prohibiting normal behaviour, the light meter is worn at a
necklace and does not induce any disturbance either. At night subjects will be
asked to wear an elastic, comfortable band around the head to measure sleep.
Rating scales and tests are distributed over time. Daily burden is low (2 min
raing scales in the morning). Only during one day each week, subjects are asked
to invest more time in rating scales and tests: they will perform the tests at
4 timepoints during the day and each testpoint will last approximately 15 min.
In addition at 4 timepoints one rating of sleepiness is collected. Subject
receive a PDA for this research purpose and this will not restrict them in
performing their normal daily life. Filling in evaluation questionnaires takes
place 4 times and last 5 min each.