The purpose of this study was to examine the effects of mild heat exposure on sleep stages and body temperature in older men. Ten healthy male volunteers with a mean age of 69.2 +/- 1.35 years served as subjects. The experiments were carried out under two different sets of conditions: 26 degrees C 50% relative humidity (RH) and 32 degrees C 50% RH. The subjects slept from 2200 hours to 0600 hours with a cotton blanket and wearing short-sleeve pajamas and shorts on a bed covered by a sheet. Electroencephalogram, electro-occulogram and mental electromyogram recordings were made through the night. Rectal and skin temperatures were measured continuously. No significant differences were observed in sleep onset latency. In time spent in each sleep stage, wakefulness was significantly increased at 32 degrees C than at 26 degrees C. The total amount of wakefulness increased and rapid eye movement sleep (REM) decreased at 32 degrees C compared to 26 degrees C. The fall in rectal temperature was significantly suppressed and the mean skin temperature was significantly higher at 32 degrees C than at 26 degrees C. These results suggest that, for older men, even mild heat exposure during the nighttime sleep period may increase thermal load, suppress the decrease of rectal temperature, decrease REM, and increase wakefulness and whole-body sweat loss.