How do fish sleep? Professional analysis of the rest and sleep mechanism of fish

Fish are aquatic vertebrates, and their physiological characteristics and behaviors are very different from those of terrestrial animals such as mammals and birds. A common question is: Do fish sleep? If so, how do they sleep? Since fish do not have eyelids, they do not close their eyes when they sleep like humans or other animals, and they do not completely lose consciousness in the water. This article will comprehensively analyze the sleep behavior of fish from the aspects of the definition of fish sleep, physiological mechanism, sleeping posture, environmental influence, and sleeping methods of different types of fish.

1. Overview of fish sleep

1.1 Do fish sleep?

From a biological point of view, fish do enter a resting state similar to sleep. While they sleep differently than mammals or birds, most fish exhibit the following when they rest:

• Reduced activity: Not moving, suspended in water, or staying in one position for long periods of time.

• Slowed reaction: Slower reaction to external stimuli, but still alert to danger.

• Reduced metabolism: Heart rate, breathing rate, and energy expenditure are reduced to restore strength.

1.2 The difference between fish sleep and mammal sleep

There are obvious differences between fish sleep and mammal sleep:

• Fish do not have rapid eye movement sleep (REM), and do not dream or completely lose consciousness like mammals.

• Fish do not close their eyes because they do not have eyelids (except for some sharks).

• Some fish will continue to swim, and even in the sleeping state, some fish still need to keep swimming to maintain water flow through the gills to ensure breathing.

2. How do fish sleep?

2.1 Sleeping posture and state

Different types of fish adopt different postures when sleeping, but generally speaking, they will choose the following ways:

• Stationary suspension: Many fish will suspend in the water and use small movements of their fins to maintain balance, such as goldfish and catfish.

• Hiding and resting: Some fish will choose to hide in corals, rocks or water plants to avoid being discovered by predators, such as clownfish, seahorses, etc.

• Lying on the bottom: Benthic fish (such as flounder and catfish) will rest close to the bottom of the water to reduce energy consumption.

• Continuous swimming sleep: Some fish that rely on water flow for oxygen (such as sharks and tuna) still need to swim slowly during sleep to ensure that the gills continue to get oxygen.

2.2 Brain activity of fish during sleep

Unlike deep sleep in mammals, the nervous system of fish still maintains certain activities during sleep:

• Some areas of the brain are still active to respond to possible predator threats.

• EEG waves during sleep show that fish enter a state of low activity but not completely unconscious, similar to the light sleep stage of humans.

2.3 Fish Sleep Time and Cycle

Fish sleep patterns are affected by day and night changes, environmental light, water flow intensity and food supply:

• Diurnal fish (such as tropical ornamental fish and goldfish) usually sleep at night.

• Nocturnal fish (such as catfish and eels) rest during the day and move at night.

• Deep-sea fish are in a dark environment for a long time, so their sleep patterns are not affected by light, but rely on biological clock regulation.

3. Sleeping patterns of different types of fish

3.1 Stationary sleep (most freshwater fish and tropical fish)

Typical fish species: goldfish, guppies, koi

• These fish will stay or suspend in a slow-flowing environment, reducing swimming to save energy.

• They enter a light sleep state at night and are easily awakened by external stimuli.

3.2 Benthic hibernation (benthic fish)

Typical fish species: Catfish, flounder, loach

• These fish are used to hiding in the mud or cracks at the bottom of the water and entering a dormant state.

• Due to the relatively hidden environment, their sleep time is usually longer, and the deep sleep time is more stable.

3.3 Continuous Swimming Sleep (Sharks and Pelagic Fish)

Typical fish species: Tuna, Shark

• These fish do not have swim bladders and cannot stay suspended in the water, so they must swim slowly to allow water to flow through their gills for oxygen.

• Some shark species (such as the great white shark) use "hemibrain hibernation", that is, one side of the brain goes into hibernation while the other side remains active to maintain swimming and alertness.

3.4 Hiding sleep (coral reef fish)

Typical fish species: clownfish, seahorse

• These fish usually hide in corals, seaweed or caves to reduce the risk of being preyed on.

• Some fish (such as parrot fish) even secrete a layer of mucus to cover themselves to cover up the smell and prevent natural enemies from finding them.

Four factors affecting fish sleep

4.1 Ambient light

Light has a huge impact on the sleep rhythm of fish:

• The biological clock of most fish is adapted to the day and night cycle, and excessive light may cause sleep disorders.

• If artificially raised ornamental fish are exposed to light for a long time, they may experience problems such as decreased sleep quality and increased stress response.

4.2 Water flow speed

• Slow water flow helps fish enter a resting state.

• Fish in a strong water flow environment (such as a turbulent river) need to adapt to a lifestyle of constant swimming and reduce the time of deep sleep.

4.3 Water temperature and oxygen content

• Water temperature that is too high or too low will affect the metabolism of fish and thus affect sleep.

• When oxygen content is low, fish may reduce sleep time to increase oxygen intake.

4.4 Natural enemies and predation pressure

• In the wild, fish sleep must be balanced with survival needs, so they do not enter a completely deep sleep to avoid becoming prey.

V. Summary: Fish sleep patterns vary by species

1. Fish do "sleep," but they do not completely lose consciousness like mammals.

2. Most fish remain still when they sleep, but pelagic fish such as sharks and tuna continue to swim.

3. Fish sleep is affected by a variety of factors, including ambient light, water flow, oxygen levels, and threats from predators.

4. Different types of fish adopt different sleeping strategies, such as suspended rest, bottom stillness, continuous swimming or hiding sleep.

Understanding the sleep patterns of fish can help us better raise ornamental fish and gain a deeper understanding of the ecological adaptability of aquatic animals. This special resting method shows the survival strategy formed by fish during evolution, which enables them to survive efficiently in underwater environments.

animal tags： fishs