Rapid-eye-movement sleep, a stage of sleep occurring in progressively lengthening episodes roughly every 90 minutes throughout the night in human adults and occupying about twenty per cent of sleeping time, strongly associated with vivid dreams (1), and characterized by rapid eye movements, theta waves, absence of delta waves, penile erections in males, and REM atonia of the skeletal muscles preventing dreams being acted out. REM sleep does not occur in reptiles but has been found in birds and almost all mammals, including dogs and cats (whose eyelids can be lifted gently to reveal darting eyes during REM episodes) and also in bats, moles, and whales, but not in the Australian spiny anteater or echidna of the genus Tachyglossus, which is exceptional in this regard. Also called active sleep, desynchronized (D) sleep or paradoxical sleep. See also REM rebound, sleep. Compare NREM sleep.