Light Timing Study

Participants will complete a 14-day protocol. On days 1-7, participants will sleep at home on a stable sleep schedule that requires 9 h of time in bed in the dark. This will ensure that participants are not sleep deprived before beginning the lab sessions. Wake time will be the average of the self-reported wake-up time (± 15 mins) before the study begins measured from pre-study sleep logs collected for a week or more. Bedtime will be 9 h before wake-up time. A device worn on the wrist that measures activity (actigraph), sleep diaries, and daily telephone messages will monitor compliance to the sleep schedule. Participants will visit the lab at least once during this week to download and review their data. If participants are not compliant to the sleep schedule, they will be dropped from the study. On days 8-13, participants will live in the laboratory for seven days. On day 8, participants will arrive at the laboratory in the afternoon, and shortly after, will begin a baseline circadian phase assessment to determine baseline dim light melatonin onset (DLMO), a reliable marker of the circadian timing system. On day 9, participants will sleep on their stable 9-h sleep schedule in the laboratory. Their 9-h fixed sleep schedule will be shifted by 1 h per day over 3 days. Participants will sleep in dark bedrooms in the lab. During scheduled wake, participants will remain awake in normal dim room light except during bright light sessions, when light will range from 5000 to 7000 lux. Study groups will differ by the light exposure they receive during the lab sessions. In the afternoon bright light (ABL) group, participants will be exposed to 3 h of bright light on 3 consecutive afternoons. On the first day (day 10), light boxes will be turned on 5 h after fixed wake-up time. Then, the start of the light exposure will move 1 h earlier each day. In the morning bright light (MBL) group, participants will be exposed to 3 consecutive mornings of 3 h of bright light. On the morning of day 11, the 3-h bright light exposure will begin 1 h before their stable wake-up time. In the morning + afternoon bright light (MBL + ABL) group, bright light will begin at the same times as the ABL and MBL groups and participants will received the same duration of bright light per day (3 h) as the ABL and MBL groups because the duration of each exposure will be 1.5 h instead of 3 h. Both bright light exposures will move 1 h earlier on subsequent days. In the room light control group, participants will complete the same 7-day lab session, except remain in dim room lighting. All participants will complete a final circadian phase assessment at the end of the 7-day lab session (day 13).