Short nights reduce light-induced circadian phase delays in humans

STUDY OBJECTIVE: Short sleep episodes are common in modern society. We recently demonstrated that short nights reduce phase advances to light. Here we show that short nights also reduce phase delays to light. DESIGN: Two weeks of 6-hour sleep episodes in the dark (short nights) and 2 weeks of long 9-hour sleep episodes (long nights) in counterbalanced order, separated by 7 days. Following each series of nights, there was a dim-light phase assessment to assess baseline phase. Three days later, subjects were exposed to a phase-delaying light stimulus for 2 days, followed by a final phase assessment. SETTING: Subjects slept at home in dark bedrooms but came to the laboratory for the phase assessments and light stimulus. PARTICIPANTS: Seven young healthy subjects. INTERVENTIONS: The 3.5-hour light stimulus was four 30-minute pulses of bright light (-5000 lux) separated by 30-minute intervals of room light. The stimulus began 2.5 hours after each subject's dim-light melatonin onset, followed by a 6- or 9-hour sleep episode. On the second night, the bright light and sleep episode began 1 hour later. MEASUREMENTS AND RESULTS: The dim-light melatonin onset and dimlight melatonin offset phase delayed 1.4 and 0.7 hours less in the short nights, respectively (both p < or = .015). CONCLUSIONS: These results indicate for the first time that short nights can reduce circadian phase delays, that long nights can increase phase delays to light, or both. People who curtail their sleep may inadvertently reduce their circadian responsiveness to evening light.