Sleep deprivation alters thermoregulatory responses. We used control of skin temperature to produce mild thermal challenge, both cool (32 degrees C) and warm (38 degrees C), and recorded esophageal and rectal temperatures, sweat rate and forearm blood flow in six healthy young women at rest. We discovered that after one night of sleep deprivation (1) both mean esophageal and rectal temperatures were reduced, (2) the mean threshold for sweating was not altered, and (3) there was no direct indication that skin blood flow was set at different levels with skin temperature neutral or cool. Peripheral vasodilation was attenuated when skin temperature was held at 38 degrees C. Following this period of mild hyperthermia, esophageal and rectal temperatures decreased much more rapidly in sleep-deprived subjects when skin temperature was cooled and held constant at 32 degrees C. We conclude that sleep-deprived women lose heat rapidly in response to a mild cooling stimulus. Sleep-deprived humans may be more vulnerable to heat loss with reduced ability to warm even at temperatures thought to be associated with thermal comfort.