Fish Sleep

Answering The Question: Do Fish Sleep?

Do fish sleep?  This is the kind of question that might flit into your mind as you dropping flakes into your aquarium in the morning or late at night when coming home, your own eyelids heavy.  Sometimes fish seem just to sit there, hiding behind the faux sunken ship in your aquarium, but their eyes look open.  Are they sleeping?  Is that what fish sleep looks like?


About Fish Sleep

You may have heard that sharks never stop moving, they keep on moving even when they are “asleep”.  This may have lead you to wonder how exactly this can work.  If you are asleep and you are moving, does that mean that you are just swimming blind?  Will a sleep shark just swim right into another fish or a wall or net without knowing it?  Do they wake up and suddenly wonder where they are?  Alternatively, do they wake up somehow, or have some sort of alarm that warns them when they are in trouble?

Well, to begin, fish do take on an activity that resembles sleep.  Just about every kind of fish we know about goes into non-active state that might be the equivalent of human sleep.  Some fish will literally stop all action and sit still.  Bass, for example, who anglers know are basically nocturnal fish, will find a nice cave-like area—say under a fallen log, and sit there not doing anything.  Many kinds of school fish will break into smaller groups and sit motionless during a certain period of the day in what seems to be a resting state as well.


For some fish, it is less clear if their lack of activity has to do with sleep or preservation.  Fish along the coral reef will often spend the day doing their business but hide at nighttime when the visibility is bad and they are in greater danger.  The vast majority of fish have some kind of an inactive period that we could consider as “sleep.”

Do Sharks Sleep?

One of the great mysteries is the question of whether sharks sleep.  Sharks, unlike other kinds of ocean dwellers, never go down into a cave, stop along the surface, and have an inactive period where they just float there.  In fact, if they did this they would die because the only way they can breathe is by moving watering through their gills by continuing to swim.

Scientists think however, that sharks may have a sleeping state when they swim straight for long periods.  This kind of line swimming is suggestive of an animal that does not have a lot of mental activity and may indeed be the shark equivalent of sleep.

Human Implications

So what is the importance of this question?  You might be surprised to learn that scientists actually don’t fully understand why we humans sleep.  From an evolutionary perspective, it doesn’t fully make sense, since you would think that being awake would be more likely to keep our ancestors from being eaten by lions.  We know however, that being able to get an adequate amount of sleep is vital to human health and mental stability, so this has lead researchers to consider whether a sleeping state is a necessary condition for the economics of mental activity, consciousness, and memory building.  In other words, perhaps our ability to rest our minds has allowed us to advance to create society and art and science.

Ironically, the answer to the meaning of sleep might partially be found by studying animals that don’t share our sleeping habits.  This would, of course, bring whole to meaning to the Soprano’s phrase, “Sleeping with the fishes.”