Changes in mean arterial pressure (MAP) and heart rate (HR) during sleep were recorded at three ambient temperatures (Ta: 16, 22 and 28°C). MAP and HR during sleep increased with lowering ofTa. The increase in MAP during the transition from NREM to REM sleep was decreased by lowering theTa. At 28°C, the average HR increased in going from NREM to REM sleep, while, at 16°C, it decreased. The coefficient of variation (CV%) of the MAP during REM sleep decreased as theTawas lowered, while that seen during NREM sleep was unchanged. This study suggests thatTahas a greater effect than sleep stage on the MAP and HR, and that MAP variability during REM sleep is greater at higher