THE NORMAL SLEEP

Abstract

Sleep is a cyclic state characterized in the human being by 5 fundamental stages, based on the electroencephalogram (EEG) pattern and the presence or absence of rapid eye movements (REM), as well as changes in other physiologic variables, like muscle tonus and cardio-respiratory pattern. The EEG shows progressive slowing with the deepening of the sleep without rapid eye movements (Non-REM) which evolves into REM sleep each 90 minutes during night sleep, comprising a predictable nocturnal sleep architecture, with definite proportions of each sleep stage. A neurochemical bio-rhythm follows sleep-wake cycle, with changes in body temperature and variations in the secretion pattern of neurotransmitters and hormones, related to different stages of sleep and wakefulness. The knowledge about physiological features and pathological variations of this complex cycle has enabled the development of Sleep Medicine and has given basis to the study of sleep disorders.