Neurobiology of sleep.

The central nervous system undergoes several dynamic changes during sleep, which are coordinated by the pons, basal forebrain areas, and other subcortical structures and are mediated by three major neurotransmitters-norepinephrine, serotonin, and acetylcholine. The neuronal populations that produce these neuromodulators constitute the central representation of the sympathetic and parasympathetic subdivisions of the autonomic nervous system. The locus coeruleus (noradrenergic) and the raphe nucleus (serotoninergic) are most active during waking and become progressively less active in the transition from non-rapid eye movement (non-REM) to rapid eye movement (REM) sleep. On the other hand, the cholinergic neurons in the dorsolateral tegmental and pedunculopontine nuclei area are active both during waking and REM sleep. Over the past decade, a number of studies have provided interesting new evidence supporting the role of sleep in sleep-dependent memory processing. These studies have been directed specifically towards the role of sleep in memory encoding, memory consolidation, brain plasticity and memory reconsolidation, and have confirmed the hypothesis that sleep contributes importantly to processes of memory and brain plasticity. It has been shown in humans that sleep triggers overnight learning on a motor-sequence memory task, while equivalent waking periods produce no such improvement. These findings have important implications for acquiring real-life skills and in clinical rehabilitation following brain trauma and stroke.