The study of the variations in different parameters of sleep–wake states following exposure to high environmental heat in three different age groups of freely moving rats have been presented in this paper. Each age group of rats was subdivided in three group (i) acute heat stress — subjected to a single exposure for 4 h in the BOD (Biological Oxygen Demand) incubator at 38 °C; (ii) chronic heat stress—exposed for 21 days daily for 1 h in the incubator at 38 °C, and (iii) handling control groups. The polygraphic, analog as well as digital sleep–wake recordings were performed just after the heat exposure from acute stressed rats and on 22nd day from chronic stressed rats. The results of this study revealed that acute exposure to high environmental heat increases sleep efficiency with significant increase in SWS (slow wave sleep) decrease in AWA (awake) time in all three age groups of rats. The increase in SWS and the sleep efficiency in these groups of rats at the cost of decreased time of AWA, indicates the involvement of the hypothalamus in thermoregulatory mechanism to conserve the energy of the body following sudden exposure to high heat. However, the reverse results were observed in the chronic stressed groups of rats, which have occurred mostly owing to the adaptations of the brain functions due to repetitive exposure to environmental heat. In consequence, the present study exhibits that the sleep is highly susceptible to the environmental heat and it is sensitive to the intensity, duration and the mode of exposure.