How can you tell if a fish is asleep? It’s in its trough.
All kidding aside, most fish sleep, sort of. The American Heritage College dictionary defines sleep as a periodic state of rest for the mind and body, in which the eyes are usually closed and consciousness is wholly or partially lost.
The thing about fish is that, unlike sharks, they don’t have eyelids, according to the National Oceanic and Atmospheric Administration (NOAA). So when they go to sleep, they do so with their eyes wide open.
Then of course there are these currents, tides, and predators they have to contend with. So how do they do it?
For starters, a large number of fish live almost motionless lives. Some float in place, some sink into a spot in the mud or coral, and some even build a nest. They’re still alert to danger, but NOAA says they’re also “dormant.”
Koi, for example, gather near the bottom of a pond at night to rest. Close examination will reveal that their fins move very slowly to maintain their position.
What about marine mammals?
Dolphins, unlike fish, do have eyelids.
According to Scientific American, bottlenose dolphins go into a deep form of sleep, especially at night. It is called logging because, in this state, a dolphin resembles a log floating on the surface of the water.
While sleeping, the bottlenose dolphin closes only half of its brain, along with the opposite eye. The other half of the brain stays awake at a low level of alertness. This is so they can still watch out for predators, obstacles, and other animals. It also tells you when to surface for fresh air. After about two hours, the animal will reverse this process, resting the active side of the brain and waking up the rested half.