Sleeping with open or closed curtains

Sleep is a basic human need and essential for good health, quality of life and
performing well during the day. The timing of sleep is regulated by a
homeostatic process (sleep pressure increasing with time awake) and circadian
process. The latter is synchronised (entrained) to the 24-hour light-dark cycle
by light. The relationship between external (social) and internal (biological)
time is called phase of entrainment. People that differ in this trait have
different chronotypes. The distribution of chronotypes is (almost) bell-shaped,
with few extreme chronotypes at both ends. Chronotype is assessed via the
Munich Chronotype Questionnaire (MCTQ) based on calculating the mid-point of
sleep on free days (MSF) and workdays (MSW), with the MSF being corrected for
sleep deficit accumulated across the workweek (MSF sleep corrected, MSFsc).
Social jetlag is quantified as the difference between MSF and MSW, and is a
marker for circadian rhythm disruption and chronic physiological stress, which
is positively correlated with smoking and body mass index (BMI). The modern
24/7-society ignores that sleep timing is individual, affecting sleep quality
and quantity, leading to sleep deprivation and directly impacting recuperation
and long-term health. About three quarters of the general population rely on
their alarm clocks on workdays, which especially concerns later chronotypes
with early work schedules. From controlled laboratory studies, there is ample
evidence showing the detrimental effects of sleep deprivation on health and
performance, with consequences that would be catastrophic in real life -
especially in e.g. shift-work occupations. These studies also show that the
thresholds for the effects of sleep deprivation are hard to identify by an
individual, meaning we need solutions that provide individuals with ways to
better acknowledge their state of sleep loss. One solution to this problem
would be to delay sleep on workdays until it matches sleep on free days,
meaning to fully taking account of an individual*s chronotype. Such strategy
would require delaying working hours, an approach of obviously little success
for plenty social reasons and constraints. The alternative solution is to
advance sleep onset on workdays using controlled light exposure, so as to allow
for longer sleep duration and to decrease the workweek daily sleep loss. We
hypothesise that a continuous earlier sleep onset across weeks and months on
workdays will result in an advance of sleep onset also on free days.

We aim to assess the effect of sleeping with open or closed curtains on sleep
quality and sleep loss and circadian phase of melatonin.

The study takes 30 days in total. During the first 14 days we ask participants
to only complete the sleep diary (less then 5 minutes). The 16 following days
participants will be asked to follow the protocol: if sleeping usually with
curtains closed then sleep with curtains open & if usually sleeping with
curtains open then sleep with curtains closed. Once per week participants also
complete the online version of the the MCTQ (e.g. every Sunday). In a sub-group
of 20 participants we will assess endogenous circadian phase of melatonin from
saliva samples and daily rest/activity profiles using actigraphy.

For 16 days sleeping with open curtains (if usually closed) or sleeping with
closed curtains (if usually open).

We do not expect any adverse effect from participating in our study.