Figure 2.

The frequency of occurrence of slow oscillations increases progressively as sleep deepens. A, The time of occurrence of the slow oscillation during the first hour of sleep in one subject is superimposed on the hypnogram. Note the increased occurrence of slow oscillations as sleep deepens (W, wakefulness; 1-4, stages of NREM sleep). B plots, for all subjects, the mean (±SD) number of slow oscillations per minute as a function of the sleep stage. Note that the rate of detection increases from stage 1 to stage 4. C reports the distribution of interdetection intervals measured during NREM sleep stages 3 and 4. The distribution peaks at 1.25 sec, indicating that the main frequency of slow oscillation occurrence during deep sleep was ∼0.8 Hz.