Ultradian rhythms in responsiveness to stimuli presented during sleep

The present study examined rhythmicity in responding to stimuli presented during sleep. Subjects were trained while awake to respond (by taking a deep breath) to an auditory stimulus that was subsequently presented after every 4 min (8 subjects) or after every 1 min (8 subjects) of continuous sleep. Visual inspection of the response data revealed, for most subjects, a tendency for response latencies to vary rhythmically with 3-5 response cycles per night. Relatively slow, "time-of-night", changes in responsiveness were also seen. Spectral analysis supported the observation of response rhythmicity. Frequency bands with centers corresponding to periods of 133, 100, and 80 min accounted for a significant proportion of the spectral variance for both the 4-min and 1-min group. Sleep was severely disrupted under the 1-min condition, but response rhythmicity was as evident as under the 4-min condition. For subjects having nearly normal sleep structure, the longer latencies in the response cycles tended to occur during nonrapid eye movement periods, while the shorter latencies tended to occur near REM. Response variability during REM often obscured cyclic tendencies. The data suggest that responsiveness during sleep