Physiological reviewState transitions between wake and sleep, and within the ultradian cycle, with focus on the link to neuronal activity
Introduction
Sleep within each ultradian cycle is a dynamic process where EEG-defined states of vigilance succeed each other in an apparently simple and well-defined manner: wake to light (stage 2) sleep to deep slow wave sleep then back to light (stage 2) sleep before entry into REM sleep or wake. As indicated later, this simplicity holds only when the data are averaged over many subjects, but it does give an overall view of the cycle structure and provides an essential reference framework for research at cellular level. While the hypnogram provides a convenient means to visualise sleep structure, and remains a useful classificatory tool in clinical practice, it has been superseded in EEG research work by more quantitative measures that bring out the essential continuity characterising the temporal progression—a continuity obscured by the assignment of discrete stages. Spectral analysis for example shows that slow waves, spindles and fast frequency oscillations co-exist at all times in sleep, in varying proportion. They take turn to predominate and it is this predomination sequence that defines stage assignment within the hypnogram. It is the detailed continuity obtained by such quantitative methods that enables conclusions to be drawn on the cellular origin of the overall structure of the ultradian cycle as seen in the spectral power time-courses.
This review is concerned with state transitions, be they in the sleep–wake cycle, within the ultradian cycle or within the NREM episode. This covers a vast field especially since over the past two decades the issue of state transition has been intensely investigated from a number of different angles, incorporating cellular, electrophysiological, physiological and behavioural studies. We therefore propose to focus primarily on those findings that enable the integration of neurophysiological data at the cellular level with time-course data at the EEG level, leaving other aspects of state transition to a brief summary and referring the reader to recent reviews where possible. This integration, made possible by the explosion in recent years of basic findings on key nuclei and neuronal circuitry implicated in the control of sleep and on the cellular basis of the different sleep rhythms,1, 2, 3 is a very challenging aspect of sleep research today. Together, these findings have provided the elements necessary for the establishment of tenable hypotheses relating the temporal progression of activity at the EEG level to that at sub-cortical level.4, 5
Section snippets
Transition from wake to sleep
The wake–sleep transition is anything but clear-cut, with a sleep latency period between the start of the polysomnography and spindle-defined sleep onset that often involves an alternation between stage 1 ‘sleep’ and wake before the occurrence of the stage 1 episode that ends with sleep onset. This episode, often referred to as the sleep onset period, marks a period of gradual change operating at every level of biological organisation and involving a progressive reduction in the arousal level
Transitions within NREM
These transitions concern the move towards deep sleep followed by a move away from deep sleep. The two together constitute the basic building block of the NREM episode, and on the average there are in the early episodes about 4 or 5 of them.4 The comportment averaged over many subjects is, however, much simpler: a single excursion to deep sleep and back (Fig. 3) and it is this simple picture that gives a reference frame for work at cellular level. At this level, Szymusiak et al. showed that the
NREM–REM–NREM transitions
NREM and REM sleep are two very different states identified by characteristic electrophysiological signatures that are detected using a combination of EEG, EOG and EMG signals. These two distinct sleep states alternate regularly across the night giving rise to about four or five ultradian cycles. It is the transitions from the synchronised NREM to the activated REM state (REM onset) and from the REM to the NREM state (REM offset) that interests us here.
Over the past decade cellular
Sleep to wake transition
Just as there exists a sleep onset period preceding definite sleep, there also exists a sleep offset period following definite sleep, often referred to as the sleep inertia period. Sleep inertia denotes a state of hypovigilance, confusion and impaired cognitive and behavioural performance, immediately after awakening from sleep. Although the sleep inertia period has received less attention than the sleep onset period, there are sufficient data to support the view that the process leading from
Acknowledgements
The Swiss National Science Foundation supported a part of the author's research. Sleep staging and signal analysis were done using the ERA software package (Phitools, Grenoble, France).*
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