Two-oscillator Model for Generating a Diurnal Change of Sleepiness: Ultradian Oscillator and the Circadian Oscillator

Abstract

Diurnal variation of sleepiness consists of daytime sleepiness increasing ca. 6 to 8 hours after waking up and nocturnal sleepiness before bedtime. Nocturnal sleepiness is associated with the circadian rhythms of core body temperature which strongly controls the central circadian pacemaker. However, the underlying mechanism involved in the source of daytime sleepiness is still unknown. Therefore, we examined the relationship between the diurnal rhythm of body temperature, autonomic nervous activities, and subjective/objective sleepiness in strictly controlled laboratory conditions. As a result, most subjects showed 2–5 h fluctuation in sleepiness. A significant correlation was found between the sleepiness, body temperature, autonomic functions in the 1–3 h after wake-up and 1–3 h before bedtime, whereas the fluctuations of daytime sleepiness were not in parallel with the diurnal changes in the temperature and autonomic functions. In sum, the ultradian rhythms during the daytime might be independent of the circadian rhythms of body temperature and autonomic function, whereas sleepiness in the morning and before the bedtime is mainly regulated by the circadian pacemaker.