Regional aspects of sleep homeostasis were investigated in mice provided with a running wheel for several weeks. Electroencephalogram (EEG) spectra of the primary motor (frontal) and somatosensory cortex (parietal) were recorded for three consecutive days. On a single day (day 2) the wheel was locked to prevent running. Wheel running correlated negatively with the frontal--parietal ratio of slow-wave activity (EEG power between 0.75 and 4.0 Hz) in the first 2 h after sleep onset (r 5 20.60; P < 0.01). On day 2 frontal EEG power (2.25--8.0 Hz) in non-rapid eye movement sleep exceeded the level of the previous day, indicating that the diverse behaviors replacing wheel-running elicited more pronounced regional EEG differences. The frontal--parietal power ratio of the lower frequency bin (0.75--1.0 Hz) in the first 2 h of sleep after dark onset correlated positively with the duration of the preceding waking (r 5 0.64; P < 0.001), whereas the power ratio in the remaining frequencies of the delta band (1.25--4.0 Hz) was unrelated to waking. The data suggest that in mice EEG power in the lower frequency, corresponding to the slow oscillations described in cats and humans, is related to local sleep homeostasis.

Environmental light has a strong impact on human physiology and behaviour, including cognitive functioning and alertness. Previous studies have shown that short light-dark (LD) cycles influence sleep in the albino rat. Rapid eye movement (REM) sleep increases after the onset of darkness and increases after light onset. In the present study, we investigated whether light affects sleep in mice. To this purpose the electroencephalogram and electromyogram of nine adult male C57BL ⁄ 6 mice was recorded under 12 : 12 h baseline LD conditions, followed by 24 h continuous darkness (DD) and 6 days with LD cycles of different durations (2 h, 30 min, 14 min, 10 min, 4 min and 2 min), presented in a randomized order. NREM sleep was evenly distributed over the light and dark intervals of all short LD cycles. REM sleep, however, was increased during the dark intervals of short (10-30 min) LD cycles. Analysis showed that in these LD cycles, the increment in REM sleep was maximal in the second minute after dark onset, where the percentage of epochs with REM sleep increased significantly to 175% of baseline values. This increase was attributable to an increase in REM sleep episode duration. The recorded responses show that sleep in mice is affected by photic stimulation. The results demonstrate that pigmented animals can show REM sleep induction after dark onset and indicate that light has significant effects on the regulation of sleep.