If you’ve ever owned a fish or gazed into an aquarium, you may have wondered – do fish actually fall into a deep sleep like humans? Can they close their eyes and snooze on their sides? As air-breathing creatures, the way we sleep looks vastly different from the way aquatic animals rest.
If you’re short on time, here’s a quick answer to your question: while fish do not sleep in the same way that land mammals do, they do rest by entering a low-activity state where they are still somewhat alert to danger but their metabolism slows down.
They do not fully lose consciousness or the ability to respond to stimuli the way humans do when sleeping.
In this approximately 3000 word article, we’ll take a deep dive into explaining how fish rest, including:
An Overview of How Fish Rest
Fish exhibit different resting behaviors than humans. They don’t experience sleep in the same way people do, but they do enter periods of inactivity and reduced consciousness that allow their bodies and brains time to rest and recover.
Fish Don’t Sleep in the Same Way Humans Do
Unlike humans, fish do not have a true state of sleep characterized by cycles of REM (rapid eye movement) and non-REM sleep. Their brains show different patterns of activity while resting compared to mammals and birds. However, their resting states still allow their bodies and minds to rejuvenate.
Some key differences in how fish rest compared to human sleep:
- Fish do not become completely unconscious or unaware of their surroundings when resting
- They do not have REM/non-REM sleep cycles
- Their sensory systems remain partially active to watch for predators
- They typically rest for shorter periods of time than human sleep cycles
So while fish need periods of inactivity, their resting differs neurologically from the sleep experienced by humans and other mammals.
Fish Enter Periods of Resting Inactivity
Though fish do not sleep like humans, they do require periods of inactivity and reduced consciousness that allows their bodies and brains time to rest and recover energy. Species like largemouth bass may spend up to 80% of their time resting.
Some ways fish enter resting states:
- Reducing physical activity and movement
- Finding a sheltered spot or returning to a home base location
- Assuming a typical resting posture (like laying on the bottom of the tank)
- Entering periods of behavioral quiescence where they are still responsive to stimuli but less reactive
The gray triggerfish, for example, finds a sheltered cavity and remains motionless with greatly reduced fin movements while resting. Cichlids may retreat to established resting sites under rocks or driftwood when entering inactive periods.
Their Sensory Systems Remain Partially Active While Resting
A key difference from human sleep is that fish keep some sensory systems active while resting to remain alert for threats in their environment. This is critical for avoiding predation.
Some examples of continued sensory activity include:
- Monitoring visual stimuli through partially open eyes
- Keeping their lateral line system receptive to detect movement, vibrations, and pressure changes in water
- Maintaining sensitivity to odors that may indicate a predator is near
So while fish brains exhibit neural patterns indicating reduced activity and consciousness while resting, they preserve environmental awareness to avoid falling completely unconscious and vulnerable like humans do during deeper non-REM sleep.
Differences Between Fish Species and Their Rest
Some Fish Rest More Deeply Than Others
Not all fish experience rest in the same way. Some species like tuna and sharks need to keep swimming continuously to breathe and don’t ever experience true rest. Other active fish like goldfish and betta fish may rest in short bursts sporadically throughout the day and night.
In contrast, less active bottom dwellers like koi can experience deep, restful sleep for longer periods of time.
Research indicates there are differences in brain wave patterns between sleeping and awake states in zebrafish, suggesting they experience various stages of rest like mammals do (source). However, most fish do not experience REM sleep characterized by dreaming.
Their resting states are lighter than human sleep but their basic rest patterns follow circadian rhythms like other animals.
Migratory Fish May Not Rest Much
Fish like salmon that swim incredible distances during seasonal migrations likely do not rest for extended periods. They need to keep moving to reach spawning grounds so resting deeply could make them more vulnerable to predators.
However, some research suggests migratory fish may be able to sleep with half their brain at a time, allowing them to keep moving while still getting some rest (source).
One study of zebrafish found that males decreased their rest during mating season in order to focus on reproduction. So migratory fish may strategically cut back on rest depending on environmental conditions and biological needs (source).
Their survival depends on balancing rest with migration and reproduction.
Bottom Dwellers Can Rest Deeply on Ocean Floors
Fish that live on the ocean floor or bury themselves in substrate like eels and rays can experience very deep, motionless rest compared to other fish. Since they don’t need to swim continuously to breathe, bottom dwellers can essentially switch off and conserve energy for hours while resting on the sea floor.
One study found that resting bright green eels lowered their heart rates and metabolism significantly compared to swimming bright green eels. This demonstrates their ability to enter a deeply restful state (source).
Their cryptic camouflage coloration also protects them from predators during vulnerable resting periods.
So while all fish require rest, species adapted to different environments and lifestyles experience rest differently. Pelagic fish need to keep moving even when resting while bottom dwellers can sleep deeply and soundly on the seabed.
How Do Fish Breathe and Move While At Rest?
Fish have a unique respiratory system that allows them to breathe and move even when they are at rest. Here’s a detailed look at how fish respiration works:
How Fish Breathe
Fish breathe through gills instead of lungs. Gills are delicate structures made up of filaments that extract oxygen from water as it passes over them. Fish continuously pump water over their gills, allowing them to extract oxygen from the water non-stop.
When fish are swimming actively, their movement helps push new, oxygenated water over the gills. But when fish are at rest, they still need to breathe. Specialized structures and mechanisms allow fish to keep breathing even without actively swimming:
- Operculum – A bony flap that covers and protects the gills. Opercula open and close to help pump water over the gills even when the fish is stationary.
- Buccal pumping – Fish can use their mouth and throat muscles to actively draw water into their mouth/throat and push it out over the gills.
- Ram ventilation – Some fish open and close their mouths to force water over their gills even when stationary. This is called “ram ventilation.”
In addition, some fish have special accessory breathing organs that help enhance gas exchange when stationary. These include:
- Labryinth organ – A highly vascularized organ in Anabantidae fish that allows them to breathe air.
- Swim bladder – Has respiratory functions in addition to buoyancy functions for some fish species.
How Fish Move When Inactive
Fish use a variety of low-energy fin movements and responses to maintain position or make slight adjustments when resting:
- Pectoral fins – Used for minor steering, hovering, or maintaining vertical position.
- Dorsal/anal/caudal fins – Gentle fin movements or positioning helps keep orientation.
- Swim bladder adjustments – Regulating swim bladder inflation/deflation allows minor buoyancy changes.
- Current-induced movement – Stationary fish may flex and orient their fins/bodies to go with water current flow.
While fish may appear completely still when resting, in reality they are almost always making minor fin and body adjustments to hold position and orientation in the water. Their specialized breathing mechanisms allow them to continue respiring even when engaging in minimal movement.
When and Where Do Fish Tend to Rest?
Nocturnal vs. Diurnal Fish Have Different Rest Cycles
Just like humans, fish need rest and sleep to function properly. However, fish don’t really “sleep” in the same way humans do. Instead, they enter periods of rest where their activity levels and metabolism slow down considerably.
Fish rest cycles depend largely on whether they are diurnal (active during the day) or nocturnal (active at night). Diurnal fish like trout and bass tend to rest more at night. They find shelter in reefs, rocks, plants or caves and become almost motionless to rest.
Nocturnal fish like catfish and eels rest during daylight hours. At night when diurnal fish sleep, nocturnal fish wake up to feed and mate.
Research shows fish need significant periods of inactivity for muscle repair and to consolidate memories from learning during their active periods. Some fish may even experience “REM-like” rapid eye movement during rest, suggesting complex brain activity occurs similar to mammalian sleep.[1]
Interestingly, some fish like tuna and sharks must keep swimming to pass water over their gills for oxygen. So they swim slowly and limit their activities to “rest”, but can’t stop moving altogether.
Sheltered Areas Help Fish Feel Secure While At Rest
Fish tend to rest in areas that offer protection and shelter from predators and ocean currents. Reef crevices, caves, dense vegetation, and sandy sea floors all provide sanctuary for resting fish.
Being near a sheltered resting spot offers fish a quick escape from threats. As social animals, fish also prefer to sleep near others of their same species, perhaps for added security and protection.
Here are some prime fish resting areas:
- Coral reefs – Reef fish like clownfish and angelfish retreat into reef holes and crevices to safely rest at night.
- Lake plants and branches – Catfish and perch hide among dense underwater plants and submerged trees/branches to avoid predators.
- Sandy/muddy bottoms – Fish like stingrays and flounder bury themselves partially in sediment, using sand to conceal themselves.
- Caves – Species like grouper and bass enter underwater rocky caves, keeping their senses alert for danger.
By resting in sheltered locations, fish reduce their chances of being eaten at their most vulnerable. The right environment helps fish relax enough to fully recharge during inactive rest periods.
Signs That Your Pet Fish Is Resting Peacefully
Their Coloring Gets Less Vibrant
When fish are active, their colors are typically bright and vibrant. This is especially true for species like bettas, guppies, and neon tetras who have shimmering fins and scales. As your fish starts to settle in for some rest, you may notice their colors become a bit duller and less intense.
This is a natural effect of the fish being in a calmer, less excited state.
They Find a Sheltered Spot and Hover
Many species of aquarium fish will find a sheltered nook or cave to tuck themselves into when they are ready for bedtime. Discus fish, for example, will often wedge themselves under driftwood or behind plants. Angelfish may retreat to the upper corners of the tank.
Cichlids are notorious for claiming their own little caves. If your fish is hovering in one spot, unmoving, there’s a good chance they are catching some Z’s.
Their Breathing Slows Down
One of the clearest giveaways that your fish is sleeping is that you’ll notice their breathing and gill movements slow down. Awake, active fish breathe more quickly. But in a resting state, they require less oxygen so their respiration rate decreases.
Watch for several slow gill movements in a row to be sure your pet fish is taking a snooze.
They Become Less Aware of Stimuli
Sleeping fish are much less reactive to things going on around them. They may not dart away or investigate if you approach the tank or make noises nearby. Many owners test if their fish are sleeping by gently tapping on the glass or dropping a few fish flakes in.
A awake fish will immediately take interest in potential food. But a sleeping fish will barely budge. This decreased awareness and sedentary behavior are reliable clues that your scaly pal is getting some quality shut eye.
Conclusion
While fish may not experience deep REM sleep where they lose consciousness like humans, they do require periods of rest where they enter a state of quiet inactivity. Different species have adapted unique ways to rest safely while still maintaining awareness of possible predators.
Understanding how fish sleep allows aquarium owners to gauge if their pet fish are getting adequate rest. Knowing fish rest cycles also allows people to marvel at the diversity of remarkable adaptations among ocean creatures.
Hopefully this guide gave you a helpful overview explaining how fish get their beauty rest under the sea!
