The transition from wakefulness to NREM sleep is associated with typical signs of brain electrical activity, characterized by prolonged periods of hyperpolarization and increased membrane conductance in thalamocortical (TC) neurons, with the consequence that incoming messages are inhibited and the cerebral cortex is deprived of signals from the outside world. There are three major oscillations during NREM sleep. Spindles are generated within the thalamus, due to thalamic reticular (RE) neurons that impose rhythmic inhibitory sequences onto TC neurons, but the widespread synchronization of this rhythm is governed by corticothalamic projections. There are two types of delta activity: clock-like waves generated in TC neurons by the interplay between two hyperpolarization-activated inward currents; and cortical waves that survive extensive thalamectomy. The hallmark of NREM sleep activity is the slow oscillation, generated intracortically, which has the virtue of grouping the other types of sleep activities, thus leading to a coalescence of different rhythms that can only be observed in intact-brain animals and humans. Far from being epiphenomena, with no functional role, NREM sleep oscillations, particularly spindles and their experimental model augmenting responses, produce synaptic plasticity in target cortical neurons and resonant activity in corticothalamic loops, as in "memory" processes. Upon brain arousal, spindles are blocked by inhibition of RE neurons, the spindles' pacemakers; clock-like delta rhythm is obliterated by depolarization of TC neurons; and the cortically generated slow oscillation is abolished by selective erasure of its hyperpolarizing components. Fast (beta and gamma) oscillations are roduced by the depolarizing effects of mesopontine cholinergic neurons acting on TC neurons and nucleus basalis neurons acting on cortical neurons.