How Do Fish Sleep at Night?

Ever wondered how your fish fall asleep at night? Fish, just like other animals, require sleep however when they take a nap it looks completely different than falling asleep in your bed at home! While it might seem like your goldfish is always swimming around the tank it needs sleep to rest and recover every other animal.

Many people do not think fish sleep because they always seem to be in motion. Fish have to keep moving even while sleeping in order to pass water through their gills and breathe. Bigger fish require more effort to move while sleeping and may have to use their tail while smaller fish like your pets at home only need to occasionally move with a fin due to water pumps and currents.

The best way to tell if your fish is sleeping is to watch for it floating, making few movements, near the bottom the the tank or near a rock or plant shelter. Scientist call this behavior suspended animation and believe fish use to to conserve energy, hide from predators and reduce their metabolism. It may look like they are dead but don’t worry fish usually don’t sleep for too long so they should be back to their normal swimming self soon.

Fish in swimming in a tank

Keep in mind some species of fish are nocturnal while others are normally awake during the day. Artificial lighting can have a big impact on the health of your fish tank, make sure you have the best natural and artificial lights for fish tanks!

There are a number of factors that can influence how and when a fish sleeps including:

  • Species
  • Tank water and lighting
  • Health
  • Diet
  • Food availability
  • Other fish in the tank
  • Environment

Fish Sleep with Open Eyes

One of the biggest differences you may notice is that fish sleep with their eyes open. Other than some shark species, fish have no eyelids to close while they are asleep. If your fish is sleeping you may notice him or her floating in a trance like state near the bottom of the tank with their eyes open.

Remember, just because a fish is sleeping does not mean they are not alert! Fish are constantly monitoring their environment for predators and are ready to dart at a moments notice! Disturbances like tapping on the glass, loud noises and the smell of food are some of the most common reasons for your fish to wake up.

How Long Do Fish Sleep?

Length of sleep in fish depends on a few factors like species, lighting and environment. Fish are able to sleep up to 12 hours per day if given total darkness and a calm environment. Large fish also seem to require more sleep, however they must also keep moving in order to breathe though their gills. Smaller fish like bettas require less rest and are able adapt to new environments and fish tanks. Fish in your home aquarium adapt to your light/dark schedule and tend to most of their sleeping at night while all the lights in the house are off. 

Fish Never Enter REM Sleep ​

Another key difference between how fish and human sleep is that fish never enter REM sleep. REM sleep, or rapid eye movement, is the phase of sleep characterized by constant, rapid eye movements with intense dreaming.

When Do Fish Sleep?

Many fish owners do not even notice their pet sleeping. A fish will quickly adapt to the lighting, pumps and environment in their tank and develop a natural sleep schedule based on their needs.

Sleep schedules for wild fish in oceans, rivers and lakes are determined by a variety of environmental factors like:

  • Lighting
  • Predators
  • Food supply
  • Temperature
  • Shelter

When it is time for sleep fish typically try to find shelter from predators in caves, holes, buried in sand or hiding in a plant. Some fish, like wrasse and parrotfish surround themselves in a mucous envelope to protect themselves at night!

Marie Seska

Marie Seska

Marie Seska is one of the most trusted sources on the net for sleep information. She searches the web - as well as the latest medical journals to bring her readers the latest news and information.

Leave a Comment