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Srrmmary.-The auditory awakening thresholds of the major electroencephalographically defined sleep stages were compared. A modification of the method of constant stimuli was used in an apparently successful attempt to minimize the incorporation of the experimental stimuli into the mental activity of the sleeper. A total of 319 experimental trials were distributed among seven human Ss who served for about six experimental nights each. The sequence and timing of experimental trials were counterbalanced to control for nights, habituation, amount of accumulated sleep, and amount of sleep since last awakening. The results showed approximately equal awakening thresholds during REM periods (the rapid eye movement stage of sleep) and stage 2 (low voltage EEG and 12 to 1 4 cps "sleep spindles"). Both these stages had lower awakening thresholds than delta sleep (large slow EEG waves). Awakening thresholds became lower with accumulated sleep, independent of sleep stage. There were no significant stage independent relationships between awakening threshold and time since last awakening or time since last body movement, although the latter were varied over a relatively narrow range which limits the generaliry of these findings. There was no stage independent relationship between heart rate and awakening threshold. The possible physiological determinants of the awakening response were discussed.During the past decade, much information has been gathered which compares psychological and physiological functioning in REM (rapid eye movement) and N R E M ( n o rapid eye movement) sleep stages; this information has recently been reviewed by Dement (1964). The present scudy aims to contribute to this body of information by comparing the sleep stages, as defined by Dement and Kleitman (1957), with respect to auditory awakening threshold. Since this study was begun, another report on the relationship becween sleep stages and awakening threshold has been published (Goodenough, Lewis, Shapiro, Jaret, & Sleser, 1965). The present scudy is nevertheless reported in detail because, not only does ic have corroborative value, but ic employs a different methodology which contributes further to a. clarification of sleep scageawakening threshold relacionships.

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