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Do fish sleep?

This is the sort of difficult question which could just as easily crop up out of absolutely nowhere as it could during a story about fish. Central to the way that children learn is the process of relating new knowledge and experience to their own life. A big part of a child’s life is sleep, so it’s a perfectly reasonable question to wonder if other things sleep like them. Most mammals obviously do; their pet dog is clearly asleep sometimes, as are Lions in the sun and horses in a stable of hay. But fish sleeping; that's an odd one. After all, have you ever seen a fish sleeping?!

The Grown-Up Answer

Fundamentally the answer is yes; fish do sleep. Over the years, there has been some debate in the scientific community about what constitutes sleep. Some definitions of sleep have been impossible for certain animals to fit, fish among them. If sleep is associated with the closing of the eyes, and inactivity in the “high processing” area of the brain called the Neocortex, fish don’t sleep. Aside from a few species of Shark, Fish do not have eyelids and can not close their eyes. Fish also function without a Neocortex. However, in general, sleep is defined using some form of a definition first proposed by a French psychologist in 1913 which said sleep consists of;

  • A species specific sleep posture; lying down for humans, sleeping in a cave or the bottom of its environment for most fish
  • The maintenance of behavioural quiescence; slowing down or becoming inactive
  • Difficulty to arose or wake up
  • Being able to reverse the sleep state; the organism can wake up again

Based on these criteria, fish do indeed sleep. The purpose of their sleeping is not 100% clear to scientists. There is no evidence that fish enter Rapid Eye Movement “REM” sleep like humans and other mammals do. It is assumed that fish sleep to facilitate their physical rejuvenation. In one study, Zebra fish were deprived of their usual sleep. They were measurably more sluggish afterwards, with mouth and gill movements significantly reduced. Compared with mammals and other animals, the variation in the ways that fish sleep is quite wide. Some species, especially larger fish like sharks, have to keep swimming whilst asleep to keep enough water passing over their gills for them to breath. The parrot fish excretes a cocoon of mucus to sleep inside. Other species will wedge themselves between rocks to keep from drifting while asleep.

Although they are not fish, it is worth giving cetaceans like Dolphins and Orcas a mention too. These sea living mammals have evolved a truly fascinating approach to sleep known as “unihemispheric slow-wave sleep”. Unable to breath underwater, these animals must consciously decide when to swim to the surface to take a breath. If they slept in the usual way, they would drown. Instead, Dolphins and their similar cousins shut down one half of their brain at a time. This happens to such a complete degree that the eye controlled by the sleeping half of their brain will shut. In this way, enough of the animal’s brain remains awake to keep them swimming, breathing and watching for predators. It is also thought that being able to keep swimming helps these warm blooded mammals to keep warm in the cold sea.

The Children’s Answer

This question is a great one for supporting children to come to an answer through their own exploration. It will be especially helpful in doing this if your setting has a fish tank which the children can observe.

Without needing to get into the complexity of the scientific debate on the subject, you can support young children to think about what it means to be asleep. Ask children to tell you what happens when they go to sleep. Listen for answers about lying down, closing their eyes, not moving, snoring and maybe dreaming. Looking at photos and videos of different fish, encourage children to observe that fish can’t sleep in the same way that they do. You can then go on to explain that fish do sleep in their own way.

If possible, allow the children to observe fish which are sleeping. This might be in an aquarium or fish tank, or in a video. Talk about how fish sleep with their eyes open, but usually become much more still. You can also talk about and look at pictures of some of the more unusual sleeping fish, such as the mucus-wrapped parrot fish!

Preschool children may also be interested to learn about how Dolphins differ from fish. Explain that Dolphins can’t breath under water like fish, and instead have to come up for air every 5 – 10 minutes. Support the children to realise that this means dolphins can’t sleep for hours at a time without swimming to the surface. Show the children video or photos of dolphins sleeping with one eye open.