Dissociation of delta EEG amplitude and incidence in rat NREM sleep

The delta (1-4 Hz) EEG of nonREM (NREM) sleep was subjected to period/amplitude analysis in 10 Sprague-Dawley rats. During NREM sleep in the 12-h light period, average delta wave amplitude and delta wave incidence (halfwaves/min) both declined; the curves were biphasic with a plateau across hours 4-6. In contrast, the behavior of amplitude and incidence was strikingly different in dark period NREM sleep. At dark onset, amplitude increased sharply and remained at this elevated level without any significant trend across the 12 hours. Delta incidence was low at dark onset and increased with a strong linear trend. These data point to several experiments to test the mechanisms mediating the behavior of delta wave amplitude at the light-dark transition; they also bear on the homeostatic model of delta sleep.