LAUSR

Late bedtimes and sleep restriction are common during adolescence, particularly on school nights. We examined circadian phase advancing effects of bright morning light (ML) with increasing doses of evening room light exposure and sleep restriction due to late bedtimes. Adolescents (n=38, 21 girls) aged 14.1 to 18.0 years completed a 14-day protocol during summer vacation. On days 1-7, they slept at home on individualized 10-h sleep/dark schedules. On days 8-14, they lived in the laboratory. On day 8, we measured their Dim Light Melatonin Onset (DLMO). On days 9 and 10, they were put to bed 1.5h (n=9), 3.0h (n=12), or 4.5h (n=8) later and kept awake in room light (104±11 lux at the angle of gaze). Wake time was unchanged; thus, sleep opportunity was shortened to 8.5h, 7h, and 5.5h, respectively. The fourth group (n=9) kept their baseline bedtime, were not exposed to any additional evening light (EL) and were not sleep restricted. On day 11, all groups were put to bed and awakened 1.5 h earlier and were exposed to intermittent bright ML (three 30-min exposures; 8599±885 lux). On each subsequent day (12 and 13), sleep and ML were advanced 1h. Thus, there were 3 days of a gradually advancing sleep schedule and bright ML. Final DLMO was measured on day 14. The 0h EL group showed a large DLMO advance (2.2±1.0 h), but the advance was reduced in the 1.5h EL group (0.5±0.6 h). DLMO delayed in the 3h EL group (-0.9±1.1 h). There was a large phase delay in the 4.5h EL group (-2.6±1.2 h). Phase shifts differed among all groups [F(3,34)=36.7, p<.001]. Circadian rhythms of adolescents were advanced when they were exposed to bright ML and a gradually advancing sleep schedule, as shown in many studies of adults. However, when kept up later in ordinary room light, the same advancing sleep schedule and bright ML did not produce as much of an advance. Phase delay shifts occurred when bedtimes were 3h or 4.5h later. EL and sleep restriction can reverse the advancing effect of bright ML. R01 HL146772 (Crowley)