That part of the sleep cycle of birds and mammals during which there is a characteristic electroencephalograph (EEG) accompanied by rapid eye (or ear) movements (REM).
The sleep cycle is made up of alternating periods of quiet sleep and active sleep. The former is characterized by a synchronized EEG and no REM or signs of dreaming. The latter is characterized by desynchronized EEG (as in the awake animal), REM, and reports of dreaming in humans, or signs of dreaming in other mammals. The periodicity of the sleep cycle is strongly correlated with the body size that is characteristic of the species. This may be due to the demands of thermoregulation.
Thermoregulatory mechanisms function normally during quiet sleep, but give no response to thermal stress during active sleep. Such lack of thermoregulatory response is much more serious for small animals, whose body temperature is more easily influenced by ambient temperature than larger animals with a greater thermal capacity. Small animals have shorter periods of active sleep than large animals. It appears that each period of active sleep is terminated before it becomes thermally risky.
Subjects: Zoology and Animal Sciences.