The popular view that behavioral quiescence is the predominant sign of sleep may be valid for the full-blown state of resting sleep, but not for the preparatory period during which many animal species display complex motor behaviors directed at finding a home for sleep. However, this aspect of behavioral immobility alone cannot differentiate sleep from wakefulness since humans and other mammals are motionless at increasing levels of vigilance, especially during expectancy and hunting conditions associated with characteristic bioelectrical rhythms. The defining signs of the period when one falls asleep are peculiar changes in brain electrical activity (electroencephalogram, EEG) produced by network operations in the thalamus and cerebral cortex. These changes are the cause, rather than the reflection, of a quiescent behavioral condition. Indeed, the brain oscillations that define the transition from wakefulness to sleep are associated with long periods of inhibition in thalamocortical cells, with the consequence that the incoming messages are blocked and the cerebral cortex is deprived of information from the outside world. Following the appearance of these initial signs, other oscillatory types mark the late stage of resting sleep and they further deepen the unresponsiveness of thalamic and cortical neurons, disconnecting the brain from the external world.

In this chapter, I discuss the neuronal properties and network mechanisms underlying the behavioral and bioelectrical signs of waking and two major sleep stages: sleep with high-amplitude, synchronized slow waves (SWS), and sleep with rapid eye movements (REM sleep).