Cold exposure and sleep in the rat: REM sleep homeostasis and body size

Study Objectives: Exposure to low ambient temperature (Ta) depresses REM sleep (REMS) occurrence. In this study, both short and long-term homeostatic aspects of REMS regulation were analyzed during cold exposure and during subsequent recovery at Ta 24°C. Design: EEG activity, hypothalamic temperature and motor activity were studied during a 24-h exposure to Tas ranging from 10°C to –10°C and for 4 days during recovery. Setting: Laboratory of Physiological Regulation during the Wake-Sleep Cycle, Department of Human and General Physiology, Alma Mater Studiorum-University of Bologna. Subjects: 24 male albino rats. Interventions: Animals were implanted with electrodes for EEG recording and a thermistor to measure hypothalamic temperature. Measurements and Results: REMS occurrence decreased proportionally with cold exposure, but a fast compensatory REMS rebound occurred during the first day of recovery when the previous loss went beyond a “fast rebound” threshold corresponding to 22% of the daily REMS need. A slow REMS rebound apparently allowed the animals to fully restore the previous REMS loss during the following three days of recovery. Conclusion: Comparing the present data on rats with data from earlier studies on cats and humans, it appears that small mammals have less of a tolerance for REMS loss with respect to large ones. In small mammals, this low tolerance may be responsible, on short-term basis, for the shorter wake-sleep cycle and, on long-term basis, for the higher percentage of REMS that is quickly recovered following REMS deprivation.