Participants completed either the evening (top) or morning (bottom) laboratory protocol on four consecutive nights to receive all four light levels (∼0.1, 15, 150, and 500 lux).

Sleep opportunities (black horizontal bars) occurred from 1:00 to 7:00 am in the evening group and from 9:00 pm to 02:00 am and 5:00 to 7:00 am in the morning group. Room lights were off (0 lux) for sleep or dimmed (∼0.1 lux) while awake between 9:00 pm and 7:30 am during in-laboratory nights. Light exposure (dotted bars) for the evening group occurred from 11:00 pm to midnight and for the morning group from 3:00 to 4:00 am.