Correlation between the circadian sleep propensity rhythm and hormonal rhythms under ultra‐short sleep–wake cycle

The aim of this study was to clarify effects of hormonal and temperature rhythms on circadian fluctuations of sleep propensity. Ten healthy females underwent 24-h sleep deprivation and entered the circadian sleep propensity assessment setting under the ultra-short sleep–wake schedule. During the experiment, sleep propensity rhythm, rectal temperature, and 24-h serum hormone profiles (melatonin, cortisol and thyroid-stimulating hormone) were investigated. The circadian sleep propensity rhythms had two apparent peaks (afternoon and nocturnal peaks) and a trough (nocturnal sleep gate). The timings of the nocturnal sleep gate and the nocturnal peak were correlated exclusively with temperature and melatonin rhythms (P < 0.05), while that of the afternoon peak was significantly correlated with habitual wake time and melatonin rhythm. These results indicate that the circadian sleep propensity rhythm is influenced not only by the circadian pacemaker, but also by sleep habit.