Short light-dark cycles influence sleep stages and EEG power spectra in the rat.

To investigate the influence of light on sleep and the electroencephalogram (EEG), chronically implanted rats were continuously recorded during a baseline day under 12-h light-12-h dark (LD 12:12) conditions, and an experimental day with short LD (LD 1:1) cycles. The percentage of non-REM sleep (NREMS) was higher and the percentage of REM sleep (REMS) lower in the 1-h light [corrected] intervals than in the 1-h dark intervals. The maximum of NREMS induction by 1-h light occurred in the habitual 12-h dark period (activity period), while the largest enhancement of REMS by 1-h darkness occurred in the second half of the habitual 12-h light period (rest period). The EEG of waking, NREMS and REMS was subjected to spectral analysis to determine the power density of the frequency components in the range of 0.25-25.0 Hz. The overall 24-h time course of the EEG-spectra in NREMS was similar under baseline and experimental conditions. Nevertheless, the spectra were modified by the short LD-cycle. In NREMS, the values in the middle and high frequencies (greater than 6 Hz in the rest period; greater than 11 Hz in the activity period) were lower in the 1-h light intervals than in the 1-h dark intervals. In contrast, activity in some frequency bands during waking and REMS was higher in the light than in the dark intervals. It is concluded that the short LD-cycle modulates the vigilance states and induces state-specific changes in the EEG, whereas circadian aspects of sleep are little affected.