A peek into nighttime at the aquarium
Most animals sleep, and fish are no different. But how water-dwellers snooze can look really different from the way humans get their zzz's. Some differences are obvious (for instance, they don’t lie down or close their eyelids, since sharks are the only fish that even have them and only close them to protect their eyes during encounters with other animals), while others are not-so-obvious (fish never go into REM sleep and some, like sharks, have to keep swimming while they snooze, because they need constant ventilation of their gills). But no matter what state sleep takes, the general idea is the same: It's a period when activity and metabolism slow way down as a way to conserve energy and restore the body.
So what does a sleeping fish actually look like? That depends on the species, but it mostly resembles a kind of daydreaming state. Some nestle into the bottom of whatever body of water they live in (like the rocks in a fish bowl or the sand in the sea) while others hover close to the bottom, barely moving except for flickering the occasional fin to stay steady. Some species become so deeply asleep that they can be lifted all the way out of the water without waking up at all. On the more exotic side of things, a fish called the parrotfish actually secretes a bunch of mucus (pleasant, right?) that surrounds it as it sleeps. (Surely, you're glad that you have a duvet instead.)
As for the specifics, the amount of time a fish sleeps depends on the species, how active they’ve been, and other environmental factors. In aquariums, fish sleep cycles are often determined by interior lights—the fish will sleep when the lights are turned off. Many fish, like minnows and coral reef fish, are active in the daytime and sleep at night while others do the opposite and are active at night instead.
Even though they are asleep, fish are still alert for danger. And their normal sleep patterns fall by the wayside during moments like migration, spawning, or when they are taking care of babies (a new mom is a new mom!). Another interesting fact? They can suffer from sleep disorders or sleep deprivation just like humans. Take zebra fish, which naturally drop their tails low and settle in at the bottom of tanks or just beneath the surface of the water to sleep. Zebra fish that are lacking something called hypocretin receptors display classic signs of insomnia (having trouble drifting off and not sleeping for as long once they do). Those same fish, if kept awake when they normally would be sleeping, catch up on their sleep as soon as they stop being bothered. Maybe the old adage should be “Let sleeping fish lie.”