Fig. 2: Experimental protocol for within-subjects assessment of light-exposure conditions.

Participants engaged in a circadian stabilization protocol for a total of 23 days, during which they visited the laboratory for a total of four times: One adaptation night (day 1) and three 32.5-h protocol visits to assess the effect of the light exposure (1 h each, starting 30 min after HBT, as indicated by the yellow boxes) conditions on the human circadian clock. The first 32.5-h visit (protocol 1 on days 8 and 9) was always the constant background (‘control’) light, while the subsequent two visits were balanced between participants (n = 16 in total, 8 women and 8 men). For the experimental visits, volunteers arrived at the lab 6.5 h before their HBT. Melatonin sampling and assessments of subjective sleepiness (with the KSS), psychomotor vigilance (with the PVT) as well as assessments of visual comfort in 30-min intervals started 5 h before HBT. Before, during and after light exposure, we also assessed objective sleepiness with the KDT. Subsequently, participants went to bed for a 6-h sleep opportunity. Following wake-up, another five melatonin samples as well as KSS assessments were obtained. During the second evening, PVT and KSS measurements were performed as on evening 1. During all nights, PSG was recorded. The figure shows the relative time for a participant whose habitual bed time is at midnight. Grey boxes indicate dim light (that is, mostly <8 lux).