The sleep habits of different species can vary according to their exposure to light, their place in the social hierarchy, their physical traits, and their environment. The adaptive sleep theory proposes that sleep evolved as a function of an animal’s environment, in order to keep animals out of danger and allow them conserve their energy when it makes sense. In support of this theory, researchers have found evidence that sleeping fish have a lowered metabolism as well as changes to their heart rate. Restorative Sleep HypothesesĪdvocates of the restorative sleep theory propose that sleep enables the body to physically repair itself, bolster the immune system, and consolidate new memories. There are two main theories regarding the origins of sleep. Scientists do not yet know why we evolved to sleep. Similarly, sleeping goldfish and perch do not wake up when touched on their tail, and sleeping nurse sharks only respond to stronger-than-usual stimuli. A number of fish species, including blueheads, Spanish hogfish, and wrasse, also exhibit periods of reduced responsiveness during which they can be easily handled and moved by scientists. Zebrafish have regular periods of time where they stop moving, and they experience a sleep rebound after sleep deprivation. Animals usually sleep more after a period when they have been deprived of sleep, which is known as a sleep rebound. Under the behavioral definition of sleep, animals exhibit temporary periods of inactivity, specific body postures that vary depending on the species, and reduced responsiveness to their environment. These brain waves are similar to slow wave sleep and REM sleep in other species, and they occur along with characteristic changes in eye movement, heart rate, and muscle tone. Fish do not have a neocortex, but researchers have identified sleep-like brain waves in another part of the brain called the dorsal pallium. In most animals, these sleep-related brain waves are typically recorded in a part of the brain called the neocortex. For these animals, sleep includes stages of slow wave sleep and rapid eye movement (REM) sleep, with distinct physical correlates such as a slowing heart rate or reduced muscle tone. Scientists have identified specific hallmarks of brain activity that define sleep in many animals, including mammals, birds, and reptiles. To determine whether fish sleep, scientists consider both brain activity and behavior.

Studying fish may reveal important answers about how sleep works, what function it serves, and how it has evolved over time.Ī sleep-like state has been observed in many fish species, including sharks, rays, zebrafish, Bermuda reef fishes, Spanish hogfish, wrasse, parrotfish, goldfish, perch, and banded knifefish. Fish are a particularly interesting case study in animal sleep, as their resting behavior has both similarities and differences to the sleep of land animals.

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