Figure 1.

Protocol for the two 5-day laboratory sessions. After 8 or 9 days of sleeping on a fixed 9-h sleep schedule at home, participants completed this protocol in the laboratory. On days 1 and 5, participants completed a circadian phase assessment in dim light (< 5 lux) to determine the salivary dim light melatonin onset (DLMO). Baseline saliva sampling began at 15:30 and ended at 03:30. Final saliva sampling began at 14:30 and ended at 08:00 the next day. An ultradian light-dark (LD) cycle occurred in the intervening 3.4 days; participants were exposed to a 4-h LD cycle with a 2-h sleep opportunity in the dark alternating with 2 h of required wake in room light (~20 lux). Participants completed this protocol of home sleep followed by the 5-day laboratory session twice in a counterbalanced order. During one laboratory session, participants received 80 mins (four 20-minute exposures) of bright white light (~5000 lux) during one of the 2-h wake periods on days 2, 3, and 4. During the other laboratory session, participants remained in room light.