VARYING PHOTOPERIOD IN THE LABORATORY RAT - PROFOUND EFFECT ON 24-H SLEEP PATTERN BUT NO EFFECT ON SLEEP HOMEOSTASIS

To assess the influence of the photoperiod on sleep regulation, laboratory rats were adapted to a long photoperiod (LPP; 16:8-h light-dark cycle, LD 16:8) or a short photoperiod (SPP; LD 8:16). The electroencephalogram (EEG) and cortical temperature (T-CRT) were continuously recorded for a baseline day, a 24-h sleep deprivation (SD) period, and a recovery day. Data obtained previously for LD 12:12 served for comparison. Whereas the photoperiod exerted a prominent effect on the 24-h sleep pattern, the 24-h baseline level of sleep and the response to SD were little affected. Recovery from SD was characterized by a marked rise in rapid eye movement sleep, a moderate rise in non-rapid eye movement sleep, and an initial enhancement of EEG slow-wave activity followed by a decrease below baseline. The amplitude and phase of the ''unmasked'' 24-h component of T-CRT did not differ between LPP and SPP. Computer simulations demonstrated that the changes of T-CRT and EEG slow-wave activity can be largely accounted for by the sequence of the vigilance states. We conclude that the photoperiod does not affect the basic processes underlying sleep regulation.