Abstract
Sleep homeostasis manifests as a relative constancy of its daily amount and intensity. Theoretical descriptions define 'Process S', a variable with dynamics dependent on global sleep-wake history, and reflected in electroencephalogram (EEG) slow wave activity (SWA, 0.5-4 Hz) during sleep. The notion of sleep as a local, activity-dependent process suggests that activity history must be integrated to determine the dynamics of global Process S. Here, we developed novel mathematical models of Process S based on cortical activity recorded in freely behaving mice, describing local Process S as a function of the deviation of neuronal firing rates from a locally defined set-point, independent of global sleep-wake state. Averaging locally derived Processes S and their rate parameters yielded values resembling those obtained from EEG SWA and global vigilance states. We conclude that local Process S dynamics reflects neuronal activity integrated over time, and global Process S reflects local processes integrated over space.
Highlights
According to traditional theory, the need for sleep accumulates during wakefulness and dissipates during sleep
The average EEG spectral power in the slow wave range in NREM sleep varies as a function of the animal’s recent sleep-wake history, and this relationship has been captured in a classical quantitative theory using the concept of the homeostatic ‘Process S’ (Borbely, 1982; Daan et al, 1984)
Process S describes a variable whose magnitude can be estimated from the level of slow wave activity (SWA) during NREM sleep, reflecting the intensity of sleep, and which is interpreted as corresponding to the homeostatic component of sleep drive (Franken et al, 1991a; Achermann et al, 1993; Huber et al, 2000a; Vyazovskiy et al, 2007; Guillaumin et al, 2018)
Summary
The need for sleep accumulates during wakefulness and dissipates during sleep. An even more fundamental question remains whether homeostatic sleep regulation reflects an active process, dynamically shaping daily sleep architecture in response to a physiological need for the homeostatic regulation of specific variables, or whether it corresponds instead to an unknown innate time-keeping process which ensures only that a certain daily quota of sleep is obtained. The earliest theories of sleep homeostasis supposed the existence of a single variable, termed Process S, which describes sleep drive at the global level (Borbely, 1982). This variable is assumed to always increase during wakefulness, independently of its content, and to decline during sleep.
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