Circadian phase determined from melatonin profiles is reproducible after 1 wk in subjects who sleep later on weekends

The aim of this study was to determine whether circadian phase from salivary melatonin profiles is the same when measured in phase assessments 1 wk apart. Eleven healthy young men and women maintained a fixed, home sleep-wake schedule, in bed, in the dark 23:00-07:00 hr on weekdays. On Friday and Saturday nights they were permitted to wake up and go to bed up to 1 hr later, and on Saturdays and Sundays they could nap between 13:30 and 16:30 hr. The study was run in the summer. Subjects wore sunglasses when outside during the day, and went outside for at least 15 min between 08:00 and 09:00 hr each morning. They maintained this schedule for 15 days before the first assessment and the 6 days in between the two assessments. During the assessments subjects remained awake overnight in < 5 lux and gave saliva samples every 30 min. A recovery nap (13:00-17:00 hr) followed the first session. The dim light melatonin onset (DLMO), offset (DLMOff) and midpoint were used as phase markers. There was minimal change in their timing between the two phase assessments. The average absolute change in midpoint (the change in phase regardless of direction) was 20 min. There was a small, 30 min delay in the DLMO. Thus, circadian phase can be measured a week in advance of any phase shifting intervention and, as long as the prescribed sleep and morning light schedule is maintained, the phase at the start of treatment can be confidently estimated.