Do Fish Sleep? All About How Fish Get Their Rest

Fish do sleep, but it doesn’t quite look the same as when you close your eyes and fall into a long, deep slumber.

“Think of it more as a ‘low power mode’ since they still need to be aware of their surroundings,” says Jessie Sanders, DVM, DABVP (Fish Practice), an aquatic veterinarian and owner of Aquatic Veterinary Services in Santa Cruz County, California.

Unlike other animals, fish don’t have eyelids, so their eyes remain open while they sleep. Once asleep, they slow down and tend to hover in a cozy spot, with their overall alertness and responsiveness to the environment decreasing slightly. This allows them to catch some R & R while still being able to react to any sudden movements or perceived threats.

Sleeping fish reduce their metabolic rate, breathing patterns, and movement. This helps them conserve energy and recover from physical exertion—just like humans and other animals do.

Most fish that you’d keep in a standard aquarium are more likely to fall asleep once it’s dark, notes Jena Questen, DVM, CertAqV, an aquatic veterinarian at Aspen Park Vet Hospital in Conifer, Colorado. She says, “It is important that fish get some periods of darkness each day so they can sleep restfully and not be kept under bright lights 24 hours a day.”

As for how long fish sleep, the duration varies between species. However, many fish rest for several hours a day, typically during periods of low light or darkness.

Just as humans have restful areas to catch some sleep, fish also prefer to rest in quiet, calm locations where they feel protected.

“What they tend to do is find a safe, secure, snug corner somewhere in the tank—without excess water flow—where they can relax and sleep without getting bounced all around in the tank by other fish or moving water,” says Dr. Questen. Common sleeping areas for fish include caves or hidden spots near plants or decorations.

This behavior is similar to that of fish in the wild, which often rest in small, sheltered crevices among plants, seaweed, and algae. These areas have lower water flow (reducing movement while they rest) and provide camouflage to help keep them out of sight from predators.

Since fish sleep with their eyes open, it can be a little tricky to know when fish are actually asleep. The more you observe your aquatic friends, the more in tune you’ll become with their patterns and behaviors.

Here are some common signs of sleeping fish, according to Dr. Sanders and Dr. Questen:

• Settling toward the bottom of the tank
• Resting in a covered or protected area
• Slow, relaxed breathing
• Hovering in place (some movement is normal)
• Resting posture (they may seem less alert or tilt against a fixture)
• Color that appears slightly duller or muted during sleep (in some fish species)

Fish are also more likely to be at rest when there is low light and reduced commotion in and around their tank.

Is My Fish Dead or Sleeping?

A common question fish owners have is whether their gilled friend is simply taking a rest, not feeling well, or has passed away. Dr. Questen says that if a fish was perfectly fine and active a few hours ago and is now sitting still in the tank, it is most likely sleeping rather than dead or ill.

Sick fish might sit still like this all day long, which is a sign that something is wrong. A fish that isn’t moving or breathing is deceased; dead fish also tend to become buoyant and float toward the top of the tank, unlike sleeping fish that settle toward the bottom.