It is well‐established that heart rate is decreased in NREM sleep relative to wakefulness, but the extent and progression of variations in heart rate when NREM sleep is first initiated, i.e. at sleep onset, have not been detailed. Furthermore, since physiological variations which have been documented during the sleep onset period have been based on studies of adult subjects, developmentally related influences on this process have not been examined. The present investigation addressed these issues by examining beat‐to‐beat (RR interval) changes in heart rate during the transitions between wakefulness, initial Stage 1, and subsequent Stage 2 sleep in normal and reading disabled male preadolescents who participated in a four‐consecutive‐night baseline sleep study. To avoid the influence of sleep deprivation or the effects of multiple sleep onset attempts, only initial, uninterrupted sleep onset periods from post‐adaptation nights were selected for study. For both groups the results indicated a significant slowing of heart rate beginning 30 s prior to Stage 1 onset, and a further decrease within 30 s of Stage 2 onset. In addition to providing new developmental data documenting heart rate variations in the wake/sleep transition, these results complement previous reports indicating motor and autonomic changes occurring in anticipation of Stage 1 onset. These data are also relevant to an ongoing controversy regarding whether initial Stage 1 or Stage 2 sleep should be considered as the time of sleep onset. To the extent that the systematic and coordinated variations across systems may be taken as an index of state change, and in the absence of remarkable differences in these variations between Stage 1 and subsequent Stage 2, the present data are most consistent with considering initial Stage 1 as the earliest EEG sign of physiological