Alterations in sleep induced by chronic exposure to mild changes in ambient temperature (Ta) were studied in male Wistar rats with chronically implanted electrodes for recording electrooculogram (EOG), electroencephalogram (EEG) and electromyogram (EMG), and a thermocouple to record the brain temperature (Tbr). Changes in sleep-wakefulness (S-W) and Tbr on exposure to warm (30+/-1 degrees C) and cold (18+/-1 degrees C) Ta for 4 weeks were studied in two groups of five rats each. Chronic heat exposure produced a persistent increase in sleep, primarily due to an increase in the durations of sleep episodes. A disproportionate increase in sleep during the dark period resulted in reduced circadian variation. The paradoxical sleep (PS)/total sleep time (TST) ratio also remained increased, during heat exposure. On chronic cold exposure, the sleep was decreased initially, but it recovered after 3 weeks, due to an increase in the frequency of slow wave sleep (SWS) episodes. The Tbr was not altered on exposure to warm Ta, but it remained high throughout the 4 weeks of cold exposure. The increase in the amount of sleep, especially the PS with enhanced ambient temperature, may be considered as an adaptation to thermal load aimed at energy conservation. Though the increased wakefulness is suggested to enable the organism to optimize thermoregulation during acute cold stress, thermoregulation itself may be readjusted to ensure homeostatic restoration of sleep during chronic cold exposure.