HEART-RATE ASSOCIATED WITH SLEEP ONSET IN PREADOLESCENTS

It is well-established that heart rate is decreased in NREM sleep rela tive to wakefulness, but the extent and progression of variations in h eart rate when NREM sleep is first initiated, i.e, at sleep onset, hav e 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 t hese issues by examining beat-to-beat (RR interval) changes in heart r ate during the transitions between wakefulness, initial Stage 1, and s ubsequent Stage 2 sleep in normal and reading disabled male preadolesc ents who participated in a four-consecutive-night baseline sleep study , To avoid the influence of sleep deprivation or the effects of multip le sleep onset attempts, only initial, uninterrupted sleep onset perio ds from post-adaptation nights were selected for study. For both group s 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 Sta ge 2 onset. In addition to providing new developmental data documentin g heart rate variations in the wake/sleep transition, these results co mplement previous reports indicating motor and autonomic changes occur ring in anticipation of Stage 1 onset. These data are also relevant to an ongoing controversy regarding whether initial Stage 1 or Stage 2 s leep should be considered as the time of sleep onset, To the extent th at the systematic and coordinated variations across systems may be tak en as an index of state change, and in the absence of remarkable diffe rences 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 sleep onset.