Do fish sleep? People with aquariums and other curious individuals often wonder about that. There is no simple “yes or no” answer. Your first paragraph …

All species have periods of rest and activity. Some secrete the same sleep-inducing hormones produced by mammals. Zebra-fish are examples.

Zebra-fish have been used in some studies to help scientists understand the roles played by melatonin and hypocretins. Melatonin makes you sleepy. Hypocretins stimulate wakefulness and energy expenditure.

In humans and other mammals, melatonin production begins as darkness falls and stops as daylight approaches. Some species of fish rest during the day and are active at night.

Most species have no eyelids. So, it is difficult to understand how they could be sleeping. All mammals close their eyes when they are asleep.

Illistration of colorful fishDo fish sleep? Many species do but not in the same way we do. Photo: Michael Lorenzo

Sharks have eyelids, but they do not close their eyes. There is a clear membrane in a shark’s eye that provides protection. It is referred to as a nictitating membrane.

Scientists believe that life began in the oceans. So, most non-plant forms of life share some things in common. The need to rest the brain is one of them. But, how do fish sleep? That depends. Here are a few examples.

While sharks never close their eyes, some species lie at the bottom of a tank and are inactive except for pumping water over their gills. The spiny dogfish or mud shark is an exception. It continues to swim while it is sleeping.

At first glance, the mud shark appears to always be active and awake. It is the mud shark’s specialized spinal cord that controls swimming. While the brain is sleeping, the spinal cord remains active.

Some species simply float in place while they are resting. Others dig holes in the mud or wedge themselves into a piece of coral while they are sleeping. Some even build a nest.

Do fish sleep like mammals? We know that they don’t close their eyes. So, that’s one difference. Another big difference is that they are still alert to danger even while they are sleeping.

While some mammals are lighter sleepers than others, they usually find a location for sleeping where they will be safe from predators. They do that because during some sleep phases they are basically unconscious. They would not be aware if an enemy approached.

While a shark may be sleeping on the bottom of the tank, his eyes will follow a diver. So, their eyes are functioning even while they are asleep.

Do fish sleep often? They seem to go through regular daily cycles of sleeping and wakefulness. They may “nap” frequently like many mammals.

The subject has not been studied extensively. They do not seem to respond to things that stimulate sleeping in humans and other mammals. They are not believed to have circadian rhythms.

There is something of a random nature to when they rest and when they are active. It seems to have more to do with feeding and hunger than with daylight or other outside influences.

This may be particularly true of species that live in the ocean depths, although that is only a theory. Studying those species in their natural habitats has been difficult.

How long do fish sleep at a time? That varies depending on the species and what they eat. Small species that survive on marine algae have longer periods of wakefulness during which time food is gathered.

Some mammals sleep for a total of 18-20 hours per day, particularly when they are young. Others like killer whales and dolphins do not sleep at all for their first month of life.

So, now you know a little bit about the subject. When someone asks, “Do fish sleep?” You can give them more information than they asked for.

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