This study will measure the effects of various sleep-wake schedules and patterns of bright light exposure on the body clock. There are no blood draws in this study; instead, we measure the timing of the body clock by taking saliva samples and analyzing them for a hormone.

If you qualify to participate, this study will involve:

• A 10-day home sleep section, where you will sleep according to a strict schedule but at times similar to your usual sleep times.
• A lab saliva collection session where you will remain awake and reclined for about 24 hours, and we will take samples of your saliva every 30 minutes. This begins on a Monday afternoon and lasts until Tuesday afternoon.
• Twelve (12) more nights of sleep at home where you will sleep according to a strict schedule but at times similar to your usual sleep times.
• Four nights in the Rush lab, where you will sleep in a private bedroom. This will happen from a Monday to a Thursday. You will need to come to the lab two hours before your usual bedtime. In the morning, we will wake you earlier than usual and expose you to bright light for a few hours. You can leave the laboratory six hours after you awaken in the morning, and will return that afternoon or evening.
• Another lab saliva collection session lasting about 24 hours. This happens on a Friday night.

The study will take about four weeks. You will participate with one other participant. Throughout the study there will also be short lab visits scheduled every two to three days. Each visit will take about half an hour. There is free parking adjacent to the lab.

To participate, you must be:

• Healthy.
• Between the ages of 18 and 45.
• Free to spend a Monday afternoon and night in the lab. You can go home Tuesday afternoon.
• Free to spend five consecutive days in the lab (Monday through Friday). You can go home Saturday morning.