Effects of Afternoon and Evening Light on Teenagers' Melatonin Levels, Alertness, Sleepiness and Sleep

Many teenagers are familiar with this: on school days, they have to get up early; during the day, they hardly get any light exposure; in the evening, they go to bed late - and are then tired at school the next day! Around the world, teenagers are sleep deprived, with studies suggesting that almost half (\~45%) suffer from inadequate sleep. Previous investigations have shown that people's sleep-wake rhythm is related to the light conditions that they are exposed to during the day and at night. However, little is known about how different light levels in the afternoon can modulate teenagers' sleep and their bodily responses to light in the late evening. Therefore, the investigators aim to study which lighting conditions have a favourable effect on these aspects and how the potentially harmful effects of light at night can be prevented.

Light exposure during adolescence seems to be the critical component of a vicious circle. Due to the maturation of sleep-wake regulatory systems in combination with progressively ill-timed exposure to light and early school start times, teenagers suffer from the accumulation of sleep depth during school days. Therefore, the proposed study investigates whether the physiological and alerting effects of late evening light exposure in adolescents depend on the intensity of light exposure in the preceding afternoon (primary endpoint: evening melatonin concentration).

The investigators aim to describe dose-response relationships, where the "dose" is the preceding (real-world applicable) afternoon light intensity (\< 10 lx, \~100 lx, or \>1000 lx EDI, 4-hour duration), and the "responses" are the adolescents' physiological and alerting responses to evening light exposure (\~100 lx melanopic EDI, 4.5-hour duration). By this route, the researchers can explore whether increasing afternoon light exposure is a feasible target for ameliorating the detrimental effects of artificial light at night and promoting healthier sleep-wake regulation during adolescence.

Eligibility Criteria:
Inclusion Criteria:

* Healthy
* Capable of judgment
* Normal BMI (Age-related Body-Mass-Index Percentile \> P3 \& \< P97; approx. corresponding to 28.5 ≥ BMI ≤ 16)
* Signed consent form of participants
* Signed consent form of a legal representative

Exclusion Criteria:

* Pregnancy or breastfeeding (only female)
* Current participation in other clinical trials
* Extreme chronotype (Extreme early or late chronotype/mid sleep time: mid-sleep time \< 1:00 / \> 7:00)
* Extremely short or long sleep durations during school- or work days (\< 6 hours \> 11 hours)
* Sleep disorders
* High myopia (\< -6 diopters)
* High hyperopia (\> +6 diopters)
* Non-normal best-corrected visual acuity (BCVA \< 0.5 \[20/40\])
* General health concerns or disorders, including heart and cardiovascular, neurological, nephrological, endocrinological, and psychiatric conditions
* Ophthalmological or optometric conditions
* Medication impacting visual, neuroendocrine, sleep, and circadian physiology
* Drug and alcohol use (urinary drug screening \& breathalyzer test)
* Non-compliance with sleep-wake times: \>1 deviation from ±60 minute window sleep and wake-up time
* Non-compliance with caffeine intake (\> 1 times caffeine intake)
* Transmeridian travel (\>2 time zones) \<1 month prior to the first session of the study
* shift work \<3 months prior to the beginning of the study