The primary objective of this study is to evaluate the hypothesis that apnea during sleep and its associated cardiac rate change is part of the causal sequence resulting in the Sudden Infant Death Syndrome (SIDS). The basic research strategy will be to determine the affect on the amount of sleep apnea of those variables known to influence the incidence of SIDS. This study will also determine the relationship between sleep apnea and central nervous system dysfunction. All infants of low birth weight (less than or equal to 2500 gm) born over a 3 year period in Onondaga County, New York will be invited to participate in this project. Respirations, heart rate, eye movements and EEG will be recorded during a sleep session within the fourth week of life. These records will be examined to determine the frequency of apnea, the interrelationship between apnea and cardiac rate, and the influence of sleep-stage on the frequency of apnea. Analyses will be performed to determine if the frequency of sleep apnea is influenced by birth weight and other infant, parental and socioeconomic variables. The significance of sleep apnea as measured at four weeks of life also will be assessed by considering its relationship to, evidence of central nervous system dysfunction as determined at 36 weeks of age by means of the Bayley Scales of Infant Development. In addition a group of low birth weight infants will be observed in the sleep laboratory within the first and again in the fourth, tenth, sixteenth and twenty-fourth week of life to establish the relationship between postnatal age and the frequency of sleep apnea.