Motor responsiveness to stimuli presented during sleep: the influence of time-of-testing on sleep stage analyses.

The relationship between time-of-night of testing (circadian factors) and motor responsiveness to stimuli presented during different stages of sleep was examined. Nine males slept for two nonconsecutive nights in the laboratory. On Night 1, tympanic temperature was assessed at 30 min intervals. On Night 2, responsiveness was assessed with an incremental series of tones presented in sleep stages 2, 3/4, and REM throughout the night. Subjects were instructed to make a microswitch closure to the tones. Results showed that for all stages, responsiveness decreased across thirds of the night. Because the distribution of each sleep stage differed across the night, the effects on responsiveness due to time-of-testing and to sleep stage were confounded. When time-of-testing was held constant, responsiveness was greater in stages 2 and REM than in stage 3/4. When time-of-testing was not held constant, effects nearly opposite of the latter could be demonstrated.