How many hours do fish sleep?

The answer is "It depends". Unlike humans, fish do not sleep in the same way, but they do experience periods of rest. Some fish enter short periods of inactivity lasting a few seconds up to a few minutes, multiple times throughout the day and night, while others have distinct sleep-like states which can last longer, such as nocturnal rest for bottom-dwelling marine fish or daily naps of a couple of hours during the daylight for some temperate water species. In general, fish do not have specialized sleep structures in their brains like we do, so their "sleep" patterns may vary widely compared to mammalian sleep.