Fish sleep like humans study explained: Fish take naps, follow sleep schedules, and may be more human than we thought
The findings, published in Nature Communications, suggest that fish sleep may be much more complex than scientists once believed.
Zebrafish go through four different sleep stages
Zebrafish, unlike humans, do not have eyelids. However, according to one report, they regularly enter periods of inactivity, during which they are less responsive to their environment.
To better understand what was happening during these rest periods, the researchers tracked the eye movements of 105 zebrafish. They discovered that fish experience four different sleep substates, and three of these stages involve eye movements, Smithsonian Magazine reports.
Researchers have previously noticed occasional eye movements in resting fish, but natural sleep has never been studied in such detail.
“That’s why I was amazed when I first saw the fish’s eyes move in such a characteristic way,” said study co-author Vikash Choudhary of the Max Planck Institute for Biological Cybernetics, as quoted by Smithsonian Magazine.
Some sleep stages of fish are very similar to human sleep
People experience various stages of sleep, including rapid eye movement (REM) sleep, during which most dreaming occurs. The zebrafish study revealed a similarly organized sleep structure. The researchers found that the fish first enter a deep sleep phase in which the eyes remain motionless. Later, especially towards the morning, fish spend more time in phases where their eyes move sideways or alternate between movement and inactivity. The fourth sleep state turned out to be something unexpected: daytime sleep.
Fish really take naps during the day
The fourth sleep substate occurred almost exclusively during daylight hours and was the most common sleep state observed.
Scientists tested whether these periods were real sleep by presenting visual stimuli to the fish. Zebrafish showed low levels of arousal, confirming that they were indeed asleep.
Researchers also found that sleep-deprived fish were more likely to enter this nap-like state after a short time, Smithsonian Magazine reports; Just like people need to take a nap after being sleep deprived.
Transparent fish gives scientists rare look inside the brain
To conduct the study, the researchers used an autonomous microscope that tracked freely moving zebrafish larvae.
Because zebrafish larvae have transparent bodies and brains, scientists were able to monitor entire brain activity in real time while observing behavior.
“This is the first time we have been able to record the entire brain of a freely moving animal,” Choudhary said, as quoted by Smithsonian Magazine.
By combining brain recordings with behavioral observations, the researchers were able to map the fish’s entire sleep architecture.
Scientists still don’t fully understand why animals sleep
Researchers say they are now interested in understanding what each sleep stage actually does.
“We’re very curious about the role different sleep stages play,” study co-author Jennifer M. Li said, as quoted by Smithsonian Magazine. “Sleep is important for many processes, from reactivating memories to clearing waste, but we don’t yet fully understand why and how this is organized over time. Zebrafish, with their transparent brains, give us a powerful way to find out.”
The study adds to growing evidence that sleep is common across the animal kingdom. Previous research has even shown that creatures without a brain, such as jellyfish and sea anemone, spend about a third of their time asleep.
FAQ
Do fish really sleep?
Yes. Zebrafish enter periods of inactivity during which they become less sensitive to their environment.
How many sleep stages do zebrafish have?
Researchers have identified four different sleep substates.
