The adolescent circadian clock entrains to social time rather than sun time

The circadian timing of sleep is an important determinant of performance and health. There are opposing viewpoints, however, on whether light entrainment of the human circadian clock is mediated primarily by social time or solar time. Here, we provide evidence that adolescents' circadian clock entrains to their behavioral light-dark cycle determined by social demands. The circadian timing of sleep was compared between school and holiday periods in adolescents (n = 112) whose sleep was either constrained or unconstrained by early school start times. The circadian rhythm of melatonin (dim-light melatonin onset [DLMO]) closely tracked adolescents' earlier and shorter actigraphy-determined sleep pattern during the school week. The phase of entrainment from DLMO to sleep (darkness) midpoint was the same between holiday and school periods. As a consequence of obtaining short sleep on school nights, however, adolescents went to sleep later and woke up earlier relative to their melatonin onset. Hence, adolescents were "biologically ready" to go to bed earlier, and they likely woke up closer to the circadian minimum of alertness compared with their holiday sleep. Our results suggest that, in modern society, social constraints on sleep and associated light exposure (natural sunlight and electrical light) play an important role in entraining the circadian clock. Key strategies that may improve the duration and circadian timing of sleep in adolescents include advancing bedtimes, stabilizing sleep across weekdays and weekends, and delaying wake-up times by starting school later.