Fish and Humans Share Surprisingly Similar Sleep Habits, Including Daytime Naps
A recent study suggests that zebrafish have four sleep substates, just like humans do—and one of them is akin to an afternoon snooze
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Even fish need their beauty rest. Just like humans, fish sleep at night—and it turns out that their sleep stages are surprisingly similar to ours.
As humans sleep, we experience four main stages. Most dreams happen in the stage called rapid eye movement (REM) sleep, when the eyes can be seen shifting around behind the eyelids. The other three stages are considered non-REM sleep, and the eyes appear still. Zebrafish don’t have eyelids to close while they snooze, but they do experience periods of inactivity, when they become less responsive to their surroundings. So, scientists wanted to study their eye movement during these times in hopes of learning more about their slumber.
In a study published in the journal Nature Communications on May 5, researchers tracked the eye movements of 105 zebrafish. They found that the animals have a complex sleep architecture involving four distinct substates, and three of those states involve eye movement.
Previous research had documented occasional eye movements in fish during their brief periods of immobility, but scientists had not investigated these movements during natural sleep. “That’s why I was astounded when I first saw the fish’s eyes moving in such characteristic ways,” study co-author Vikash Choudhary, a computational and systems neuroscientist at the Max Planck Institute for Biological Cybernetics, says in a statement.
Fun fact: Dolphins sleep with one brain hemisphere at a time
Dolphins’ breathing isn’t automatic—they must consciously control it—so the marine mammals don’t sleep the same way we do. Instead, dolphins sleep by allowing one half of their brain to snooze while the other remains alert—so they appear to shut just one eye while napping.
To conduct the study, Choudhary and his colleagues used a self-driving microscope that followed larval zebrafish as they went about their lives. Zebrafish larvae have transparent bodies and brains, so the researchers could also capture their neural activity in real time. “This is the first time we can record the whole brain of a freely moving animal,” says Choudhary to Sara Novak at the New York Times.
By combining the brain recordings with behavioral observations, the team identified four sleep substates in zebrafish. In the first, the fish is in its deepest sleep, and its eyes are not moving. Then, toward the morning, it spends more time in the second substate, where its eyes jerk to the side. In the third state, the eyes still move sideways, but they have longer periods with no movement.
The fourth substate is more of a daytime nap. It was the most frequent of the sleep states, but it occurred nearly exclusively during the day. The researchers presented the fish with visual stimuli during this period and found that they had low arousal, which indicated that it was a true sleep state. What’s more, when a fish was deprived of sleep, it was more likely to take these naps in a short window after the deprivation.
“We are very curious about what roles the different sleep stages play,” says Jennifer M. Li, a study co-author and systems neuroscientist at the Max Planck Institute for Biological Cybernetics, in the statement. “Sleep is important for so many processes, from reactivating memories to waste clearance, but we don’t yet totally understand why and how this is organized in time. Zebrafish, with their transparent brains, give us a powerful way to find out.”
Michael Heithaus, a marine ecologist at Florida International University who was not involved in the recent study, tells the New York Times that it’s thrilling that scientists are starting to understand different sleep states.
While most animals sleep, the reason why they do so has remained a mystery. A study published in January found that even jellyfish and sea anemones—creatures without brains—spend about one-third of their time asleep, like humans do. The work suggested the behavior may have partly evolved to reduce DNA damage in nerve cells.
“It’s like a lot of things in biology,” Heithaus adds to the New York Times. “The more we look under the hood, the more complex it is.”
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