All participants will go through all three light conditions in the three experiment sessions: They will receive white fluorescent overhead light (given in melanopic EDI at eye level) as the 4h afternoon light intervention. In the first experimental session, they receive an intensity of >1000 lx. In the second experimental session, they receive an intensity of ~100 lx, and in the third experimental session, they receive an intensity of <10 lx.

All participants will go through all three light conditions in the three experiment sessions: They will receive white fluorescent overhead light (given in melanopic EDI at eye level) as the 4h afternoon light intervention. In the first experimental session, they receive an intensity of >1000 lx. In the second experimental session, they receive an intensity of <10 lx, and in the third experimental session, they receive an intensity of ~100 lx.

During the "Moderate" light condition, the four-hour afternoon light exposure at the participants' eye level will be dim (~100 lx melanopic EDI). In the 4.5-hour evening light exposure, this will constitute a light intensity of ~100 lx melanopic EDI at the participants' eye level.

During the "Bright" light condition, the four-hour afternoon light exposure at the participants' eye level will be dim (>1000 lx melanopic EDI). In the 4.5-hour evening light exposure, this will constitute a light intensity of ~100 lx melanopic EDI at the participants' eye level.

The investigators will examine slow-wave activity (SWA; delta power density between 0.5 and 4.5 Hz) during the first sleep cycle. EEG slow-wave activity (SWA) (i.e., delta power density between 0.5 and 4.5 Hz) will be calculated as an indicator of sleep propensity across the night within each non-rapid eye movement NREM part of a sleep cycle.

The investigators will score the PSG-derived sleep stages and arousals according to the American Academy of Sleep Medicine (AASM) Manual for the Scoring of Sleep and Associated Events.

The investigators will assess subjective sleepiness using the single-item 9-point Karolinska Sleepiness Scale (KSS) - a well-validated, highly sensitive subjective Likert-type measurement scale for subjective sleepiness.

Scores range from 1 to 9 with higher values on the scale corresponding to higher sleepiness.

Objective alertness will be measured using a modified auditory Psychomotor Vigilance Test (aPVT). After a response, the next tone will be played randomly after 2-10 s. The reaction time data will focus on mean 1/reaction time (mean 1/RT), the most sensitive measure for a slight deviation in sleep pressure. Mean 1/RT will be calculated after the removal of false starts and lapses.

The investigators will measure changes in the pupil area using silent substitution pupillography and examine the differences between melanopsin response amplitude before the afternoon light condition (pre-light treatment) and the melanopsin response amplitude after the afternoon light condition (post-light treatment).

Skin temperature will be continuously monitored with six surface temperature thermocouples placed on proximal and distal regions of the body surface. Skin temperatures (distal & proximal) and the distal-proximal skin temperature gradient (DPG) will be calculated.

The volunteers will perform a Karolinska Drowsiness Test (KDT) three times during scheduled wakefulness. During the KDT, participants fixate on a point on the wall from a one-meter distance for five minutes (eyes open). These sessions will provide EEG data with relatively few artefacts. As the first indicator for objective sleepiness, EEG-derived alpha/theta ratio will be calculated.

The volunteers will perform a Karolinska Drowsiness Test (KDT) three times during scheduled wakefulness. During the KDT, participants fixate on a point on the wall from a one-meter distance for five minutes (eyes open). These sessions will provide EEG data with relatively few artefacts. As the second indicator for objective sleepiness, electro-oculogram-derived (EOG-derived) slow-eye movements will be calculated.

Compliance to regular sleep-wake cycles over three weeks will be monitored with the actimetry device starting five days before the first experimental session (baseline + adaptation night) and ending with the final session.

An adapted German version of the first six items of the Visual Comfort Scale (VCS) will be used to assess the participant's visual comfort under the different lighting conditions. Additionally, participants will rate their momentary affect and well-being in relation to mood, hunger, relaxation, and motivation. Items are rated on a 7-point Likert-type scale (1-7) with higher values corresponding to a higher manifestation of the characteristic (for instance well-being, the brightness of the light etc.)

Volunteers will report their sleep and wake episodes using a sleep-wake diary (like a questionnaire).

In the mornings after the laboratory sleep, participants will additionally fill in a sleep quality questionnaire (Leeds Sleep Evaluation Questionnaire; LSEQ). The LSEQ is a 10-item, subjective, self-report measure that includes a visual analogue scale, where every item is scored from 0 to 10 where 10 corresponds to a higher manifestation of the characteristic (for instance tiredness).

In the mornings after the laboratory sleep, participants will additionally fill in a dream recall questionnaire (Sleep Mentation Questionnaire) which addresses numerous characteristics of dream recall, such as the number of dreams, emotionality, vividness, pleasantness, hostility, and colourfulness, on a Likert-point scale (1: greatly, 2: fairly, 3: little, and 4: not at all). Higher values on the scale correspond to a lower frequency of dreams.