Diurnal preference, sleep habits, circadian sleep propensity and melatonin rhythm in healthy human subjects.

After 24-h sleep deprivation, 33 healthy young subjects entered the 10/20 min ultra-short sleep-wake schedule for 26 h. Melatonin rhythm was hourly assessed simultaneously. Results indicated that morning preference was significantly correlated with habitual sleep onset (r=-0.41, P=0.04), habitual sleep offset (r=-0.52, P=0.002), melatonin peak time (r=-0.36, P=0.04), and sleep propensity onset time (r=-0.36, P=0.04). The intervals between habitual sleep mid-point and melatonin peak time and between habitual sleep mid-point and sleep propensity onset time were significantly longer in morning-preference subjects than in evening-preference subjects (P<0.05). These findings suggest that the variance of diurnal preference may be related to differences in phase relations between habitual sleep timing and the circadian pacemaker.