Concepts and models of sleep regulation: an overview

Abstract

S U M M A R Y Various mathematical models have been proposed to account for circadian, ultradian and homeostatic aspects of sleep regulation. Most circadian models assume that multiple oscillators underlie the differences in period and entrainment properties of the sleep/wake cycle and other rhythms (e.g. body temperature).Interactions of the oscillators have been postulated to account for multimodal sleep/wake patterns. The ultradim models simulate the cyclic alternation of nonREM sleep and REM sleep by assuming a reciprocal interaction of two cell groups. The homeostatic models propose that a sleep/wake dependent process (Process S ) underlies the rise in sleep pressure during waking and its decay during sleep. The time course of this process has been derived from EEG slow-wave activity, an indicator of nonREM sleep intensity. The predictions of homeostatic models have been most extensively tested in experiments. The interaction of Process S with a single circadian process can account for multimodal sleep/wake patterns, internal desynchronization and the time course of daytime sleepiness. Close links have emerged between the processes postulated by the various models and specific brain mechanisms. Due to its recent quantitative elaboration and experimental validation, the modelling approach has become one of the potent research strategies in sleep science.KEYWORDS circadian, homeostatic, mathematical model, nap, ultradian O V E R V I E W OF T H E M O D E L SThree basic processes underlie sleep regulation: (1) A homeostatic process mediating the rise in 'sleep pressure' during waking and the dissipation of 'sleep pressure' during sleep; (2) a circadian process, a clock-like mechanism defining the alternation of periods with high and low sleep propensity and being basically independent of prior sleep and waking; and (3) an ultradian process occurring within sleep and represented by the alternation of the two basic sleep states nonREM sleep and REM sleep. The three processes are illustrated schematically in Fig. 1. The tables provide a synopsis of models that have been proposed to account for circadian, ultradian and homeostatic aspects of sleep regulation. The mathematical equations of the major models are compiled in the appendix. Models of circadian rhythms (Table 1)The main objectives of these models are (1) to simulate spontaneously occurring phenomena in free-running Correspondence: Professor A. Borbkly, Institute of Pharmacology, University of Zurich, Gloriastrasse 32, CH-8006 Zurich, Switzerland rhythms such as changing phase-relations between the sleep/wake cycle and the temperature rhythm, and the splitting of the rest-activity rhythm into two components; and (2) to account mainly for the effects of light, the major zeitgeber, on the phase, amplitude and period of circadian rhythms. The database of the models consists in general of records of rest and activity rather than of polygraphically recorded sleep and waking. Moreover, the models attempt to describe general features of the circadian system rathe...

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