Short nights attenuate light-induced circadian phase advances in humans

Abstract

Context: In humans, sleep duration often determines the night (dark) length experienced, because we close our eyes when we sleep and are exposed to artificial or natural light when we are awake. Although it is recognized that there is an increasing trend in modern society toward shorter sleep time, it is not known how short nights (long photoperiods) affect the human circadian system.

Objective: In this study we investigated for the first time the effects of night length on circadian phase shifts to light in humans.

Design and setting: Eight young healthy subjects experienced 2 wk of 6-h sleep episodes in the dark (short nights) and 2 wk of long 9-h sleep episodes (long nights) in counterbalanced order. After each series of nights, they were exposed to four 30-min pulses of morning bright light (approximately 5000 lux) that advanced by 1 h/d for 3 consecutive days while night (dark) length was maintained at 6 or 9 h. Circadian phase was determined from the circadian rhythm of melatonin in dim light before and after the 3-d bright light treatments.

Results: The phase advance in the melatonin rhythm during the short nights was less than half of that observed during the long nights (P < 0.05).

Conclusions: This result shows for the first time that people who curtail their sleep may unwittingly reduce their circadian responsiveness to morning light. This finding also demonstrates that sleep length can alter human circadian function and has important implications for enhancing the treatment of circadian rhythm sleep disorders.

Fig. 1

An experimental protocol for an individual who experienced the short nights first. Black shading shows sleep/dark episodes at night and optional nap zone during the day during the short nights. L, Advancing bright light stimulus: four 30-min bright light pulses alternating with ordinary room light, starting 8 h after the baseline DLMO, and advancing by 1 h/d; D, dim light (<60 lux) in the laboratory; ↑, time of DLMO; ↓, time of DLMOff. The sunbursts indicate 10 min of outdoor light. For clarity, the phase assessments are shown as starting and ending at 1200 h.

Fig. 2

An individual’s melatonin profiles during the baseline (○) and final phase assessment (●) during the short and long nights. The protocol for this individual is shown in Fig. 1. The DLMO advanced only 0.5 h during the short nights, but 2.0 h during the long nights. The DLMOff advanced only 0.8 h during the short nights, but 1.9 h during the long nights.

Fig. 3

The mean phase advances observed in DLMO, midpoint, and DLMOff due to the advancing bright light stimulus during the long nights (■) and the short nights (□). Error bars represent the SEM. There was at least a 2-fold reduction in the phase advance in all three circadian phase markers during the short nights compared with that during the long nights.