Fig. 1: Experimental crossover protocol for assessing the AEE light interventions.

Adolescent volunteers participated in a 21-day study schedule that included ≥ 5 days of sleep-wake stabilisation with sleep and light monitoring before each of the 3 experimental visits, which were individually scheduled to begin 8 h before HBT and last until 10.5 h after HBT. On average, HBT (0:00 in the protocol) corresponded to 22:15 (SD = 31 min). AEE light interventions lasted from 7.5 h to 3 h before HBT and varied between 'dim', 'moderate' and 'bright' light exposure in a counterbalanced order with illuminance levels of ~6.5 lx, ~130 lx, and ~2500 lx ( ~4 lx, ~80 lx, ~1400 lx mEDI). This was followed by later evening light exposure from 3 h before to 1.5 h after HBT (130 lx illuminance; ~80 lx mEDI). While awake, toilet breaks were scheduled every 1.5 h, and 4 equicaloric meals were provided every 2 h. We obtained half-hourly samples of subjective sleepiness (from 7 h before HBT) and saliva (from 6 h before HBT) until 1 h after HBT, plus a final evening sample taken 1 h and 20 min after HBT. 2 morning samples were taken 5 and 35 min after wake-up. PVT and VCS were administered on arrival as baseline, then 5 times during the protocol at 90 min intervals and the last time 1 h after HBT. Starting 7 h before HBT, skin temperatures were continuously monitored at 60 s intervals. PLR measurements, polysomnography (including KDTs), skin temperature after bedtime and sleep-related questionnaires taken in the morning were not included in this article. Abbreviations: AEE afternoon to early evening, exp. experimental sessions, HBT habitual bedtime (for home sleep), KDT Karolinska Drowsiness Test, KSS Karolinska Sleepiness Scale, mEDI melanopic Equivalent Daylight Illuminance, PLR pupillary light response, PVT Psychomotor Vigilance Task, VCS visual comfort and well-being scales.