Do fish sleep? Fish don’t sleep as humans do. Instead, fish rest by significantly reducing their activity and lowering their metabolism. This resting process provides the fish with recuperation similar to sleep while remaining somewhat alert to potential danger.
The truth is that scientists are still studying many fish species to understand our finned friends better. There are tens of thousands of fish species living in fresh and saltwater all over the world. That’s a lot of studying!
In people, scientists see distinctive patterns of electrical activity in the brain. However, fishes don’t have complex brain structures to generate these patterns.
Fish don’t sleep in the truest form, but most would agree that other criteria can be used to define a sleep-like state for fish species. So we could call this ‘fish sleep’ if we wanted to, and I know you do! You are crazy like that!
- A period of prolonged inactivity or even motionless
- Consistent resting posture in a shelter (such as a crevice in the reef)
- A day/night rhythm between a period of high arousal and low arousal to the fish’s environment over a 24-hour period
For convenience, I will use the words sleep and rest pretty loosely throughout this article since most pet owners project their ‘people’ traits to all animals. So we naturally think ‘sleep’ when we have already covered that fish have a rest period that isn’t actually human-style sleeping.
How can you tell if a fish is asleep?
Fish activity varies from species to species. Fish will greatly reduce their activity down to a range of movement from slight movement of fins or body to virtual stillness. This is easy to spot when compared to a normally active fish. A sleeping fish will be less attentive to its environment and more difficult to startle. This activity, combined with hiding in coral or rock formations, is a good sign the fish is probably sleeping.
Fish are also likely to hover closer to the bottom, protective rocks, or even the water surface. However, it is difficult to tell when a fish species that is a cave dweller or hoverer is resting as the movement may be similar between awake and resting.
Do fish swim while they are sleeping?
Most fish need to keep some movement going, even while they are resting. For fish to breathe, they have to keep water moving through their gills. Some fish will have a slight body and fin movement to keep oxygenated. This slight movement also helps the fish keep stable and in its sleeping spot.
Some fish actually swim during their resting period. Pelagic fish such as tuna, bluefish, Atlantic mackerel, and bonito are always swimming. This is true of some sharks as well. They need to keep slowly swimming to keep water moving through their gills. There is quite a diversity of sleep in the fish world!
There is also a term known as ‘sleep swimming.’ This describes sleep where the fish are motionless in crevices or coral branches of the reef except that their fins are constantly moving to keep their sleep area well oxygenated.
Even though they are actively fanning their fins to create water movement, they still are resting enough to not respond to stimuli such as light or predators close by.
How much sleep do fish need?
There are a lot of variables. Fish rest is necessary to help the fish recuperate and maintain proper immune function to fight off diseases and pests. Fish under stress will often require more sleep, even taking some ‘power naps’ throughout the day.
Do fish sleep sideways?
Most fish species sleep upright. Some fishes that lay on the bottom, including those that bury themselves to some degree, may rest leaning against an object. However, if you see a fish that appears to be sleeping on its side or leaning sideways, the fish is likely ill.
Fish under stress and without sufficient sleep can lose the immune function necessary to fight off diseases and pests. This can cause a fish to be overcome by an illness that a healthy fish could ward off.
A disease of the fish’s swim bladder can cause fish to be sideways. Just like people, fish need rest to stay healthy!
Do fish sleep upside down?
Fish do not sleep upside down. Like fish being sideways or leaning in the water, an upside-down fish is likely suffering from a fish’s swim bladder disease.
An ill fish will often exhibit problems swimming too.
Do fish need darkness to sleep?
Most species of fish are used to a day and night cycle. Most species will sleep during the night cycle and some during the day cycle. The swell shark, for example, sleeps during the day.
Those with home aquariums should be sure to give their fish plenty of nighttime opportunity to sleep. Keeping it quiet and dark is necessary for the fish to get an essential resting period to stay healthy.
Here’s a link to my article on how to set up a fish-healthy home aquarium.
Some fish, such as goldfish, can change their habits from night sleepers (diurnal: active during the day) to sleeping in the day (nocturnal: active during the night) depending on food availability. If food is more readily available at night, then the goldfish often change to being awake at night to get at the goodies.
Fishes that live in almost total darkness
Even fish in the deepest depths that live in perpetual nighttime have a sleep/rest cycle. The absence of a day and night cycle doesn’t change the need for healthy rest for the fish.
Fish that don’t sleep!
There are periods within a fish’s life where they sometimes decide not to sleep. For example, many species of fish are active day and night during the spawning season. Some fish parents stay awake day and night for several consecutive days, tending to and protecting their eggs. These fish parent of the year awards go to species like the convict and rainbow cichlid, damselfish, and bass.
Additionally, some fish stay awake day and night during periods of migration.
Summary
Indeed, fish don’t sleep in the truest form, but most would agree that other criteria can be used to define a sleep-like state for fish species.
- A period of prolonged inactivity or even motionless
- Consistent resting posture in a shelter (such as a crevice in the reef)
- A day/night rhythm between a period of high arousal and low arousal to the fish’s environment over a 24-hour period
So, fish don’t sleep like humans, but they have sleep-like behavior that is just as necessary to the fish as sleep is to humans. The National Ocean Service of the National Oceanic and Atmospheric Administration agrees with me, so I must be correct! 😉