Normal sleep and its neurophysiological regulation

Normal sleep consists of two states: NREM (light and deep sleep) and REM, alternating in a cyclical pattern. The sleep/wake rhythm is regulated by two processes: the sleep propensity, building up during wake, and the circadian rhythm, imposed by the suprachiasmatic nucleus. The arousal pathways in the brainstem regulate the wake state. The ventrolateral preoptic nucleus in the hypothalamus (VLPO) is important for the control of NREM sleep. The VLPO neurons and the arousal pathways mutually inhibit each other, resulting in the alternation between sleeping and waking. Orexin from the lateral hypothalamus may stabilize the switching between these states. Cells in the pons and mesencephalon bring about the main phenomena of REM sleep. Although the regulation of the alternation between REM and NREM is not yet clear a mutually inhibitory interaction has been proposed between NREM active (REM-off) neurons and REM active (REM-on) neurons in brainstem and midbrain.