Fish don’t sleep like humans. They don’t have eyelids to close, and they don’t lie down. Instead, they have a resting phase. During this time, they might hover in one spot or slowly swim around. Their bodies and brains take this time to recover. In humans, the region of high cognition, the neocortex, hits the snooze button for seven to eight hours (a healthy sleep dose). However, fish have no neocortex. The only studies on how the fish brain functions when a fish sleeps are sourced from zebrafish. Zebrafish have transparent skin that allows scientists to see their individual brain cells at work under a microscope.
While humans enter a deep sleep with rapid eye movement (REM), fish do not experience REM sleep. However, some fish, like sharks, keep moving during their rest because they need to push water over their gills to breathe. Not all fish are the same; some rest at night, while others rest during the day.
Scientific studies on zebrafish, a popular research fish, found that they sleep for about 4 to 6 hours a night. During this time, their brain activity slows down, similar to how our brains calm down when we sleep. And just like a tired kid, if you keep a fish awake when it wants to rest, it will try to “catch up” on its sleep later.
In short, while fish don’t tuck into beds, except for parrotfish, which secretes a bag of mucus within which it stays while sleeping to protect itself. Some fish can also anchor themselves to coral reefs to prevent getting swept away.
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