Cerebral circulation and sleep

Abstract

Already Aristoteles discussed on restorative functions of sleep (cf. [1]). BorbeÂly [2] was the ®rst to show that the restorative function of sleep correlates to a reduction of homeostatic pressure built up during the day that can be assessed by the slow wave activity of the sleep electroencephalogram (EEG). The observation of a fronto-occipital power gradient in the EEG slow wave activity [3] points towards the existence of topographical differences in homeostatic functions of sleep. In accord with these electrophysiological data, recent studies with various techniques including positron emission tomography (PET) [4±6], functional magnetic resonance imaging (MRI) [8, 9] and laser Doppler owmetry (LDF) [10, 11] have demonstrated profound changes of cerebral blood ow (CBF) in various brain structures throughout the sleep-wake cycle. Based on the assumption that CBF is closely coupled with cerebral metabolism in the healthy brain these perfusion changes are thought to re ect modi®cations in the tissue metabolic state. Three review articles in this journal summarize our present state of knowledge about cerebral hemodynamics during sleep in healthy humans [12] as well as in patients with sleep disordered breathing [13] and cerebrovascular diseases [14]. In their review, Zaccoli et al. [12] ®rst summarize several studies (mainly with PET) demonstrating absolute and region-speci®c CBF changes during wakefulness, NREM and REM sleep as well as throughout the night [4±6]. While recognizing divergences in the results of these studies, the authors stress the