Five young adult males slept two consecutive nights under each of the five ambient temperatures chosen within the usual range: 13 degrees C, 16 degrees C, 19 degrees C, 22 degrees C, and 25 degrees C. Bedding and other ambient parameters were kept constant under all five ambient temperature conditions. The average REM cycle length significantly decreased when the ambient temperature increased from 13 degrees C to 25 degrees C. Other REM sleep characteristics such as total duration of REM sleep, average REM period, and REM sleep latency did not significantly differ from one ambient temperature condition to another.