Do Fish Sleep?

Yes, some fish do have a sleep-like state, although the brain activity is different from that of humans and other mammals. Can it truly be considered sleep? It depends on your definition, but it is evident that many fish enter a resting, semi-dormant state that appears to have some restorative function.

The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

Sleep is “a state characterized by a reduced responsiveness to sensory stimuli, suppressed locomotor activity and rapid reversibility to wakefulness,” according to Nature. It’s the brain’s nightly maintenance shift, an essential phase for clearing waste, consolidating memories, and rebooting the body for peak performance.

In humans, it is divided into rapid eye movement (REM) sleep and non-REM (NREM) sleep, which show markedly different patterns of brain activity. NREM consists of three stages — N1, N2, and N3 — with N3 often referred to as slow-wave or deep sleep. REM sleep is most strongly linked to vivid dreaming, though people can experience dreams in NREM stages.

Do fish sleep? Yes, probably. 

Some fish have a very similar behaviour, although it's not identical. In 2019, scientists at Stanford University set up a mini-aquarium and recorded the brain and body activity of zebrafish. The tiny tropical fish appeared to enter a state of dormancy resembling sleep, even though their eyes remained open (understandably, since they lack eyelids). 

By analyzing their brain activity, the researchers identified two distinct sleep-like phases – “slow-bursting sleep” and “propagating wave sleep” – which they believe have clear similarities to the slow-wave sleep and REM sleep seen in mammals.

Given the evolutionary difference between humans and fish, the study indicates that sleep evolved in animals at least 450 million years ago, before any creatures crawled out of the ocean.

Do some fish not sleep?

Comparable patterns of sleep-like brain activity have been observed in other fish species. However, not all fish exhibit such states. 

Many pelagic species that inhabit the ocean’s midwaters, as opposed to the surface or the seabed, appear to swim continuously. For these fish, constant movement is essential. By keeping water flowing over their gills, they can extract the oxygen they need to survive, leaving little opportunity for prolonged inactivity.

Does that mean their brain is constantly on the go with no sleep-like rest? Perhaps, but not necessarily. 

Some fish might be able to chill out while still moving by swimming together in tightly coordinated groups. Schooling lets each fish reduce how much sensory information it has to process, like cruising on autopilot, achieving one of the key benefits of sleep without closing their eyes or resting. Essentially, being part of a synchronized group may give them some of the restorative or efficiency advantages of sleep while staying alert.

It should be noted that fish are incredibly diverse, with well over 30,000 known species, yet it's estimated that fewer than 10 fish species have been studied for sleep behavior. This means our understanding of how fish sleep, or avoid sleep, is still very limited. It’s safe to say that science has only just begun to explore the fascinating and largely mysterious world of snoozing fish.