Gender Difference in Timing of Nocturnal Rise of Subjective Sleepiness

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Experimental research of diurnal variations in body temperature and melatonin secretion consistently revealed an earlier entrained circadian phase in women than in men. Since it is well documented that daily fluctuations in self-reported level of sleepiness closely follow the circadian rhythms of melatonin and body temperature, one can predict that gender differences in phase characteristics of the 24-hour fluctuations of subjective sleepiness resemble the differences revealed by research of physiological and hormonal rhythms. Analysis of sleepiness self-scored by 46 male and 54 female participants of sleep deprivation experiments showed that female participants scored significantly higher their midnight sleepiness level. The model-based simulations of sleepiness curves confirmed the prediction of a relatively earlier phase of 24-hour oscillations of sleepiness in women. Such gender differences persisted after accounting for individual variation in habitual sleep times and morning-evening preference. In today’s environment, the earlier circadian phase in women can cause a larger delaying phase shift in response to midnight exposure to artificial light, but, on the other hand, the earlier rise of subjective sleepiness can make them less vulnerable to the delaying shifts.

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