Figure 1.

Protocol to measure the free-running circadian period in adults and adolescents. After 8 or 9 days of sleeping on an individualized fixed 9-h sleep schedule at home, participants completed a 5-day laboratory protocol. On days 1 and 5, the salivary dim light melatonin onset (DLMO) was measured in < 5 lux. An ultradian light-dark (LD) cycle occurred in the intervening 3.4 days; 2-h sleep/dark (black rectangles) alternated with 2 h of required wake in dim room light (~20 lux). The ultradian LD cycle produced forced desynchrony, which allowed the circadian system to free run. DLMO phase shift from baseline to final phase assessments was used to calculate the free-running circadian period.