Do all animals sleep?

Abstract

Some animals never exhibit a state that meets the behavioral definition of sleep. Others suspend or greatly reduce 'sleep' behavior for many weeks during the postpartum period or during seasonal migrations without any consequent 'sleep debt.' Rats die from one form of sleep deprivation, but sleep loss has not been shown to cause death in well-controlled studies in other vertebrate species. Some marine mammal species do not show evidence for REM sleep, and convincing evidence for this state in reptiles, fish and insects is lacking. The enormous variation in the nature of rest and sleep states across the animal kingdom and within the mammalian class has important implications for understanding the evolution and functions of sleep.

Figure 1.

Light sleepers: animals that show little or no sleep during migrations, in the postpartum period or throughout their lives. (a) Commerson’s dolphin; (b) bottlenose dolphin Tursiops truncatus; (c) and (d) killer whale Orcinus orca being born; (e) bullfrog Rana catesbeiana; (f) white-crowned sparrow Zonotrichia leucophrys. Rana catesbeiana photo courtesy of James Harding; killer whale photos courtesy of SeaWorld, San Diego.