A hypothesis has been advanced that the primary function of rapid eye movement (REM) sleep is to provide periodic endogenous stimulation to the brain which serves to maintain minimum requisite levels of CNS activity throughout sleep. REM, in effect, is the mechanism used by the brain to promote and ensure recovery from sleep. Failures of REM may hinder, or in extreme cases, prevent arousal from sleep. The possibility that two fatal sleep-associated syndromes (Sudden Infant Death Syndrome and the Oriental Nocturnal Death Syndrome) may involve deficits of REM was discussed.