This review describes the systemic physiological phenomena characterizing the interaction between thermoregulatory and sleep processes in the adult mammal. Homeostatic thermoregulation is preserved across the behavioral states of quiet wakefulness and non-rapid eye movement sleep notwithstanding state-dependent differences in threshold and gain of effector responses to thermal loads. In many mammalian species rapid eye movement sleep is characterized by the suppression or depression of thermoregulatory responses to thermal loads. In human adults, however, rapid eye movement sleep is not as thermally altered as in other mammals. The experimental evidence shows that the interaction between thermoregulatory and sleep processes occurs at the level of the preoptic-hypothalamic thermostat. A main open question concerns the nature of the over-riding demand imposing on the central nervous system the temporary suspension of homeostatic integrative regulation in rapid eye movement sleep.