Do fish sleep?

Answer

Dear Tim,

Not an easy question! The answer depends on what you mean by “sleep”. Since fish don’t have eyelids, they can’t close their eyes like we do when we sleep. In addition, when we sleep, our brain exhibits a certain nervous activity in an area of ​​the brain called the neocortex. Fish do not have such a developed neocortex as we do and therefore do not show the typical brain activity that we have when we sleep. So, on the grounds that fish cannot close their eyes and show no typical “sleep brain activity” either, it can be argued that fish do not sleep.

Yet this answer is a bit too simple, because if one defines sleep rather as a state of rest characterized by (1) a (strongly) reduced metabolism, (2) a (strongly) reduced physical activity, (3) a (strongly) reduced response to certain environmental stimuli and (4) adopting a “resting position”, then there are many fish species that do indeed “sleep”. The best example of this are the parrotfish: these fish find a resting place at night (usually a cavity between corals or in rocks) and then cover themselves with a slime layer (a kind of “blanket” if you like!). They spend the night there almost motionless and in that state they react very weakly to external stimuli. The next morning, they will crawl out of their slime casing and cavity to go swimming normally again. Another example is tilapia: in aquariums, these fish “rest” on the bottom at night and show lower respiration rates while also having little or no response to external stimuli.

Many other fish, however, show no sleeping behavior at all. For example, tuna and some sharks never stop swimming around. In such cases it is not possible to speak of sleep, of course. Even blind cave fish never seem to sleep.

In short, the answer to your question is a bit of ambiguity. Yes, some fish seem to have a behavior that we can describe as a form of sleeping, but some other fish we can almost certainly say that they have no sleeping behavior.

Kind regards,
Thierry Backeljau