How Do Fish Sleep?

How Do Fish Sleep?

Yes fish sleep. But it’s not sleep as we know it. They don’t have eyelids to close, they sometimes do it during the day, they don’t show the characteristic brainwave patterns like REM sleep seen in humans, and some, including most sharks have to keep swimming in their sleep.

But fishes do have a period of reduced activity and metabolism which seems to perform the same restorative functions as nocturnal sleep does in humans. Some are more obvious about it than others and actually rest on the bottom or in coral crevices, and parrotfish secrete a mucus “sleeping bag” around themselves before they go to sleep. If you get up quietly in the middle of the night you will find your goldfish in an almost trance- like state, hovering near the bottom of the tank making just the minimum correcting motions with its fins to maintain its position in the water column. If you put food in when they’re like this they take noticeably longer than usual to respond, as if they have trouble waking up.

Mr. Oliver Crimmen, Fish Curator, Department of Zoology, The Natural History Museum

How can you tell if a fish is asleep?

In human terms, they don’t have the type of day / night lives that we recognise on the old 9 to 5 routine. Some fish are active by day, others by night. Many fishes live quite still lives on a regular day / night cycle. (Deep down it’s pretty dark anyway!)

Some deep ocean fish appear never to stop moving (often said of some sharks who have to keep water and oxygen flowing through their gills).

Most fish do rest. A bit of ’shut eye’ is difficult for most fish as they don’t have eyelids except for sharks. Most fish ’switch off’ when safe to save energy and go into daydream mode. You can see this in a fish tank or aquarium – some float motionless in place, some float near the bottom of their tanks, some build nests or nestle themselves into the seabed or coral for safety and camouflage.

They might look but like most resting animals are alert for danger whilst resting or “sleeping” saving energy. What exactly is going on in their brain (and human type sleep / brain waves) would be harder to tell. I’ve no idea if they dream either. However a recent bit of research (featured in a Times article in October 16 2007) suggest they even get sleepless nights if you keep annoying them!

“Researchers have now been able to show not only that [zebra] fish sleep, but that they can suffer from sleep deprivation and insomnia. By repeatedly disturbing the fish using mild electric shocks, researchers were able to keep zebra fish awake at night. Those fish that had suffered a disturbed night were found to catch up on their sleep as soon as the opportunity arose.”

Mark Norris, Education Officer, Newquay Zoo

sleeping fish

do fish sleep?