To date human brain temperature has not been measured exactly. Limited published data indicate it to be about 37.5 degrees centigrade, which surprisingly is 1.O degrees centigrade lower compared with placental mammals larger than the rat. Although the human brain is only 2 percent of body mass, it accounts for 20 percent of basal metabolism. Therefore, the removal of excess heat produced inside the brain is the main problem for its temperature regulation. The brain-arterial blood temperature difference in humans is probably twice that of larger mammals - 0.5 degrees centigrade. These two temperature factors play a crucial role for human brain homeothermy, particularly during motionless quiet waking and sleep. Low ambient temperature causes sleep deprivation. Moderate ambient heat allows sleep with negligible disturbances, and in humans induces sweating on the face and on the hairy (or bald) skin of the head. In passive hyperthermy human brain homeothermy depends on: (i) sweat evaporation from the skin surface of the face and whole head with face skin vasodilation, and (ii) enhanced venous return from the skin to the sinus cavernosus. This sinus is situated ventrally to the hypothalamus. Tympanic temperature reflects brain temperature fluctuations in humans.