The development of temporal organization of sleep-wake states during the preterm period was examined. Seventy-one high-risk preterms from two cohorts were observed from 7 to 11 p.m. weekly from the time they were no longer critical until discharge. Mixed general linear model analyses found that with increasing postconceptional age, quiet waking, active waking, and sleep-wake transition bouts occurred more frequently, quiet sleep bouts occurred less frequently, and active waking and quiet sleep bouts increased in length. However, these developmental patterns were not stable over cohorts. On the other hand, the transitional probabilities between states were similar in both cohorts, providing evidence for biological bases for some aspects of temporal organization. Active sleep was pivotal in state transitions. Younger infants showed fewer transitions that did not involve active sleep. Thus, temporal organization is an early characteristic of sleep-wake states but is not a unitary phenomenon. Transitional probabilities remain relatively invariant, whereas bouts lengths and frequencies may be altered in different populations or by differing environmental conditions.