How do fish, dolphins, and whales sleep?
Fish rest by slowing down and hovering or hiding, eyes open because they have no eyelids. Dolphins and many whales let only half the brain sleep at a time, because they have to stay alert enough to swim up and breathe.
Fish rest with their eyes open
Fish have no eyelids, so they sleep with their eyes open. They slow down, hover in place or tuck into a crevice, and react more slowly. Some even wrap themselves in a slimy cocoon for the night.
Share this stepDolphins sleep with half a brain
A dolphin lets one half of its brain sleep while the other half stays awake, then switches. The eye controlled by the awake half stays open to watch for danger and keep track of the group.
Share this stepThey must remember to breathe
Unlike us, dolphins and whales don’t breathe automatically. Every breath is a choice, made at the surface through the blowhole. A fully sleeping brain could mean drowning, so part of it stays on duty.
Share this stepWhales take short naps
Big whales rest near the surface or hang still in the water. Sperm whales have been seen sleeping upright in groups, in naps of just 10 to 15 minutes, and they sleep less than almost any other mammal.
Share this stepSleep, without forgetting to breathe.
Resthalf-awake brainsafe sleep
Some birds use the same half-brain trick, sleeping with one eye open on long flights. Sleeping in shifts lets an animal rest and stay safe at the same time.
Common follow-up questions
Do sharks have to keep swimming to breathe while they sleep?
Some do, because they need moving water over their gills. Others can pump water over their gills while lying still on the bottom.
Do baby dolphins sleep?
Studies of dolphin mothers and calves suggest both stay active almost around the clock for the first weeks, so the calf can breathe and keep up.
Do fish dream?
No one knows. Fish don’t seem to have the sleep stage linked to vivid dreams in humans, but some show sleep-like brain activity.
How long do whales sleep?
Studies are hard, but many whales seem to sleep only a few hours a day, in short naps, often near the surface.
