The short answer
Yes — aquarium fish do sleep, just not the way mammals do. They drop into a restful, low-activity state, usually at night, going still and less responsive while keeping their eyes open (they have no eyelids). It’s completely normal, so a fish resting quietly in the dark is a healthy sign, not a problem.
What fish sleep looks like
Resting fish hover in place, settle on the substrate, or tuck into plants and decor. Their colours may fade, their breathing slows, and they react less to movement — some look almost lifeless until the lights come on. Different species rest differently: many are active by day and sleep at night, while nocturnal fish such as some catfish and loaches do the reverse and rest during the day.
Give them a proper day–night rhythm
Fish rest best with a consistent light cycle. Put your tank light on a timer for a steady period each day and switch it off at night so fish get a reliable dark rest — leaving it on around the clock stresses them and feeds algae. Avoid startling sleeping fish by turning a bright room light on suddenly in a dark room; a gentle transition is kinder. Plenty of cover and plants give shy fish somewhere secure to settle.
When rest looks like a problem
Normal sleep is quiet and the fish resumes activity when the tank wakes up. It’s different if a fish is listless during the day, lying on the bottom with clamped fins, breathing hard, or off its food — those are behaviour changes worth checking. Start by testing the water with a liquid test kit, and compare with why is my fish staying at the bottom? and how do I know if my fish is sick? if you’re unsure.