Sleep regulation in rats during early development.

Sleep states and power spectra of the electroencephalogram (EEG) were determined in freely moving young rats. Recordings during 24 h were obtained from the same animals at three different ages. Already at the age of 23 days waking predominated in the 12-h dark period. Rapid-eye-movement sleep (REMS) declined between the age of 23 and 40 days. Its 24-h maximum was situated in the dark period at 23 and 29 days of age and in the light period at 40 days. Slow-wave activity (SWA; 0.75-4.0 Hz) of the non-REMS (NREMS) EEG showed marked age-related changes: a declining trend in the 12-h light period was absent at 23 days, moderate at 29 days, and prominent at 40 days. At 23 days, SWA progressively declined within ultradian sleep episodes and at 24 days was massively increased after 2-h sleep deprivation (SD). At the age of 30 days, 6-h SD induced much smaller changes. The distinct 24-h pattern of high-frequency activity (10.25-25.0 Hz) was present at all ages and may represent an EEG correlate of a circadian process. We conclude that homeostatic mechanisms regulating NREMS intensity are already operative a few days after weaning.