It's a token memory of childhood — that time every year at your school's fundraiser festival where you play a game to win a prize goldfish. You carry it home suspended in a clear plastic bag while your parents silently wonder how long it will last.
A few days later, you walk over to your new fishbowl and see your aquatic pet upside down, motionless. In your youthful curiosity, you wonder, is that how fish sleep?
It's hard to tell when a fish is sleeping because they don't have eyelids (or ones that close, if you're a shark). But scientists know that fish rest, and some have some very interesting ways of doing it.
How Do Fish Catch Z's?
Fish don't sleep the same way that you, your dog — or any mammal for that matter — does. Most don't experience REM cycles (though research on zebra fish suggests they have similar brain activity to REM sleep).
Still, scientists have observed that many species of fish rest; this is a period of restoration when their metabolic functions slow and they are less active. Some fish, like reef sharks, lay still at the bottom of the ocean or inside caves when they sleep. These sharks have a unique anatomical feature called a spiracle that forces water out of the shark's gills so they can continue to breathe. While they are resting, they are not as responsive to their surroundings.
Some species of sharks and tuna must keep swimming to breathe because they don't have spiracles to keep water flowing over their gills. In their cases, scientists hypothesize that they shut off half of their brains like dolphins do. They slow their breathing and move more slowly but are still somewhat responsive to their environment.
Like an incorrigible snoring sibling, some fish species like Spanish hogfish are known to sleep very soundly. So soundly, in fact, that divers can touch and move them to the surface without disturbing their slumber. Others have to create their own protection. Parrotfish, for instance, creates enough mucus to form a cocoon around themselves at night when they stop moving. The mucus blanket is likely used to keep blood-sucking parasites from clinging to them while they sleep. Clown fish bury themselves in sea anemones to protect themselves from predators while they sleep.
Fish Have Biological Clocks, Too
Like humans, fish have biological clocks that tell them when to be alert and awake and when to rest, so not all fish sleep at night. Some fish are nocturnal, and some are diurnal (awake during the day, like many humans). These tendencies primarily evolved based on when they are more or less likely to be attacked by predators or what the best times are to find a meal.
One of the more bizarre sleeping habits of the ocean is experienced by loaches, which float to the top of the water and remain completely still when they sleep for short periods. So, if your fish is a loach and it's motionless at the top of its tank, it very well could be sleeping!
Unfortunately, the same can't be said about that pet goldfish, which tends to rest toward the bottom of its habitat (not upside down near the surface).