After preliminary experiments had determined that the night sleep (unclothed and uncovered) was subjectively undisturbed from 27 to 36°C, night sleep recordings were made on 10 healthy young men. 4 hrs before and after sleep, performance tests were made in constant room temperature. The subject slept at room temperatures of 27, 31 and 36°C in the climate chamber and were studied with polygraphic sleep recordings. The following results were obtained: The lower room temperature, the higher ther percentage of deep sleep and REM sleep was. Warm nights had higher percentages of wakefulness and higher latency of REM. Subjectively at 36°C no subject was 'refreshed', whereas 6 of the 8 students were 'refreshed' after sleeping at 27°C. Movements during sleep increased at higher temperatures and decreased at lower temperatures. This behavior is useful in terms of thermoregulation. Night sleep at 27°C leads to the greatest loss of body warmth (mean: 42 cal/m2), especially from the core of the body and increased of noradrenalin in the urine. The room temperature also excretion of the heart rate in all sleep stages. The minimal mean was found to be 52/min during the REM stage at 27°C. From the follow up observations, a carry over effect of the room temperature was noted in the morning. The excretions of 17 corticosteroids and total corticoids in the urine were decreased after the 36°C nights.