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Do Fish Sleep?

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    Fish don’t sleep in a bed like you do. But they do sleep! (Krieg Barrie)
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    Most fish don’t have eyelids, so it’s hard to tell when they are sleeping. (AP/Domenico Stinellis)
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    The parrotfish wraps up in a mucus cocoon and sleeps hidden in coral. The unusual “sleeping bag” protects the parrotfish from predators and parasites. (123RF)
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    Sharks must keep swimming to breathe, even while they sleep. (NOAA via AP)
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    Dolphins and whales breathe air through blowholes on the tops of their heads. Half of the animal’s brain stays awake so it remembers to come up for air while sleeping. (AP/Armando Franca)
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From bettas to sharks, God created more than 35,000 species of fish. With gills and scales, fish are different from people in many ways. But scientists say that like humans, fish need sleep.

Sleep is one of God’s gifts to His beloved creation. (Psalm 127:2) Nearly every living thing needs some kind of rest: Plants become inactive at night; fields recover after lying fallow; even creatures without eyes or brains exhibit sleep-like behavior.

Scientists are just beginning to understand sleep’s many benefits. Expert Michael Twery says sleep “affects growth and stress hormones, our immune system, appetite, breathing, blood pressure, and [heart] health” in humans.

Some of those same benefits may apply to fish, whose sleeping habits prompt ongoing research.

Like humans, fish seem to have sleep cycles. Many are active during the day and rest at night. Some, like some eels, rays, and sharks, switch things up and take the nightshift. Aquarium fish snooze between seven and 12 hours each day.

Most fish don’t have eyelids, so their eyes don’t close. But during their rest, they stop swimming and remain very still. Their metabolism slows and their gills pump less as they doze.

Sleeping people are mostly unaware of their surroundings—even if some awaken at movement or loud noises. But most napping fish stay aware enough to detect predators.

Sometimes fish sleep in open water. More often they’re near the bottom of whatever body of water they inhabit. Fish also prefer squeezing into a spot near rocks or plants so predators can’t attack and currents can’t whoosh them away.

Sharks and some others species of fish must swim constantly to breathe. If they stop swimming, they die. How do they sleep if they’re always moving? Instead of stopping altogether, sharks simply slow their swimming, or swim into a current to sleep.

Whales and dolphins aren’t fish. They’re mammals. They spend their lives in the ocean, but they can’t breathe underwater. Instead, they must rise to the surface occasionally and take in air through blowholes on top of their heads.

If they went into a deep sleep, whales and dolphins would drown. They wouldn’t come to the surface to breathe. So they sleep by resting just half their brains at a time! The other half remembers to rise to the surface, breathe, and stay alert enough to spot danger.

Scientists wonder whether some fish might do the same thing. But they don’t know. There is much more to learn about how fish sleep.

Why? The animal kingdom reminds us that God gives many good gifts—like sleep—to all of His creation.