How Do Fish Sleep? | By Sara Le
Every creature on earth has some way of resting and so do fish. Sleep, if defined as a time when rest is taking place, eyes are closed, and there is no thought or moving around, is something that happens in fish. Because most fish do not have eyelids, they don’t close their eyes as such, but they do go through a period when their bodies slow down and they are not going anywhere. It might be in reality more of a rest than a sleep but to the fish it has the same effect as a night’s sleep has on humans. It is also why in aquariums, lights are turned off so that the fish sleep at least a portion of every day.
No one knows why fish sleep. It might be because, as in people, their bodies need to have a recovery phase, or it might be because many fish hide away to sleep, that they use it to protect themselves from predators and regain energy at the same time. Scientists have studied how fish sleep or rest and they have discovered several different things.
Fish are known to have times when they are resting and are not aware of what is going on around them. However, they do not go into the deep REM sleep that humans do. Some fish spend time in-between being asleep and being awake. Some kinds of fish sleep by going into a type of hibernation. Take, for example, the Koi fish. This fish, like many others, reacts to the coming of cold winter temperatures by going down to the water bottom. They slow their metabolic rate and cease activity. With Koi fish, their digestive system even closes off so they do not get hungry. When temperatures warm up, they emerge back into their normal living depth.
Other types of fish become dormant during hot and dry weather periods instead of cold ones. Some fish sleep in the mud. Carp are one example of this behavior, as is the African lungfish. Lungfish often cover themselves with mud and leave a little hole through which to breathe air. One of the most interesting of all examples of fish sleeping is one of the gobes that lives in the Ganges River. These fish are amazing! They dig a burrow and sleep in it during the driest months. They breathe through their tail, which is the only part of their body still touching water.
Scientists have discovered a parrotfish that lives on reefs who sleeps by squeezing himself into crevices. Then he excretes mucus which covers his entire body with a protective coating and goes to sleep. Some sharks never really stop moving at all, as they have to be in motion to push water through their lungs so that they can breathe. They are definitely in some kind of state of rest but there is no way to tell if it is what we would define as sleep.
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