Abstract The present research was concerned with the nature of an individual's responsiveness to extrinsic stimuli during sleep. To address this general problem, sleeping individuals were presented different auditory frequencies which were equated for subjective loudness in phons but were not equated for physical intensity. The prediction was that if an individual's responsiveness during sleep was similar to that during waking, then the auditory stimuli should produce equal levels of arousal. If, however, an individual's responsiveness during sleep was not similar to that during waking, then, because the stimuli were not equated for physical intensity, the auditory stimuli should produce different levels of arousal. The results suggested that while the waking psychophysical relation between auditory frequency and subjective loudness adequately predicted an individual's response during sleep characterized by fast wave patterns of EEG activity and no REMs, it did not reflect an individual's responsiveness during sleep characterized by slow wave patterns of EEG activity. In fact, during slow wave sleep, the subject's level of arousal appeared to be directly related to the physical intensity of the auditory stimuli.