Periods of sustained sleep were analyzed to assess the development of sleep-state organization and structure during the first six months of life. Infants first establish consolidated sleep and wakeful periods, which then become oriented to the 24-hour day/night cycle. As infants mature they gradually show greater sleep 'efficiency', and at onset of sleep, REM periods become less likely. The longest sleep period progressively becomes associated with the dark period of the 24-hour cycle. This study also assessed the sequences of NREM-REM sleep-cycle organization during the first six months of life. Although individual infants may have a significant correlation between duration of sleep and latency at onset of sleep, the over-all direction is not constant, nor is an 'age effect' apparent during the first six months. The data support the hypothesis that states of alterness , which develop independently in the perinatal period, become integrated in early infancy.