A model-based interpretation of the biphasic daily pattern of sleepiness

We developed a thermoregulatory model of sleep control based on the hypothe sis that non-rapid eye-movement sleep participates in homeostatic thermoreg ulation. This model successfully reproduced several qualitative features of human sleep/wake cycles during entrained as well as the internally desynch ronized states. Among the reproduced features, generation mechanisms of the biphasic sleepiness distribution are studied here in the light of the mode l structure. Harmonic analysis is employed for this purpose. Through linear izations and confining the harmonics of the masking process to the fundamen tal component, a simplified representation of sleepiness is obtained. The s implified sleepiness is constructed with the fundamental circadian, the sec ond harmonic components, and the constant (DC). The bimodality of the sleep iness is shown to be made by the second harmonic which is added to the fund amental component. The behavior of their amplitudes and phase positions are investigated under the Varied sleep/wake durations and phase differences b etween the oscillators. Since the sleepiness generated by our model is roug hly mimicked by the simplified representation under diverse conditions, thi s simplification can be regarded as adequate. From the behavior of the cons tituents of respective harmonic components, the fundamental component is sh own to originate from the sleep/wake masking process and the circadian osci llators; the second harmonic from the multiplicative interactions between t he circadian oscillators and the sleep/wake masking process. These results indicate that the rhythmic processes are principal constituents of the slee piness, at least in the steady state.