While fish do not sleep in the same way that land mammals sleep, most fish do rest. Research shows that fish may reduce their activity and metabolism while remaining alert to danger. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest.

How do you know if a fish is sleeping?

It's pretty easy to tell when fish are sleeping: they lie motionless, often at the bottom or near the surface of the water. They are slow to respond to things going on around them, or may not respond at all (see some sleeping catfish here). If you watch their gills, you'll notice they're breathing very slowly.

Do fish swim when they sleep?

Most fish need to keep moving even when they're sleeping, so that they keep a constant flow of water moving past their gills to maintain a proper oxygen level in their bodies. For some larger fish, like sharks, this can take the form of swimming at a slower rate when sleeping.