The cardiorespiratory control system undergoes functional maturation after birth. Until this process is completed, the cardiorespiratory system is unstable, placing infants at risk for cardiorespiratory disturbances, especially during sleep. The profound influence of states of alertness on respiratory and cardiac control has been the focus of intense scrutiny during the last decade. The effects of rapid-eye movement (REM) sleep on various mechanisms involved in cardiorespiratory control are of particular significance during the postnatal period since newborns spend much of their time in this sleep state. In fullterm newborns, REM sleep occupies more than 50% of total sleep time, and this percentage is even greater in preterm newborns. From term to six months of age, the proportion of REM sleep decreases. Since respiratory and cardiac disturbances are known to occur selectively during REM sleep, the predominance of REM sleep may be a risk factor for abnormal sleep-related events during early infancy. Awareness of these developmental changes in sleep patterns is important for clinicians dealing with problems such as apparent life-threatening events (ALTE), sudden infant death syndrome (SIDS), and/or cardiorespiratory responses to respiratory disorders. Our current understanding of respiratory and cardiac control rests mainly on studies conducted during the first months of life. There is a paucity of data on late infancy and early childhood. The present paper will review available data on how sleep affects 1) ventilatory mechanics, in particular of the upper airways and the chest wall; ventilation and apnea; gas exchange; chemoreceptor function; and arousal responses; 2) changes in heart rate and heart rate variability, and the occurrence and mechanisms of bradycardia.