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• Distinct sleep states observed in fish for the first time across the circadian cycle
• Three sleep types with eye movement, one type without
• Results made possible by observation of freely swimming larval fish with tracking microscope
Humans and other mammals cycle through distinct sleep phases. One of them is easily recognized by the darting motion of the eyes behind closed lids, giving it its name: REM (rapid eye movement) sleep. This is the state in which we experience our most vivid, intense dreams.
Whether animals other than mammals similarly experience distinct sleep states—and, more narrowly, sleep characterized by eye movements—has long been subject to debate. Researchers at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, have now discovered a complex sleep architecture in fish for the first time. Zebrafish (Danio rerio), like most teleost fish, are known to undergo periods of inactivity lasting several minutes, during which they are less responsive to stimuli. The study, now published in the journal Nature Communications, shows that these extended stretches of quiescence are not all alike: The research team identified three types of sleep, each with its own distinctive eye movement pattern, plus a fourth type with no eye movements at all.
Tracking sleep in freely swimming fish
Previous studies had noted occasional eye movements in fish during brief periods of immobility, but no one had systematically characterized them during naturally occurring sleep. “That’s why I was astounded when I first saw the fish’s eyes moving in such characteristic ways,” says Vikash Choudhary, who is—together with Charles Heller—first author of the study. The success may be owed to a new experimental setup, Choudhary explains: “What sets our approach apart is that we were the first to simultaneously record eye and body movement over a full 24-hour period in freely swimming fish.” This was made possible by a specialized tracking microscope that follows larval zebrafish as they move around freely. Zebrafish larvae have transparent brains, allowing the microscope to capture whole-brain neural activity in real time.
Combining brain recordings with behavioral observations, the team observed that each state follows its own circadian rhythm. Sleep without visible eye activity predominates at night. Of the three quiescence states with eye movement (QEM), one peaks at night, while another increases toward morning. Most strikingly, QEM-1, the most frequent of the QEM states, occurs almost exclusively during the day. This is all the more surprising given that fish in QEM-1 are especially hard to wake, leaving them vulnerable to predators.
A conserved sleep architecture
For this daytime sleep phase, the team also observed that brain-wide activity was largely reduced. Along with other neural and behavioral hallmark features, this confirms that QEM-1 is indeed a sleep state, akin to a daytime nap. The brain recordings also revealed predictable activity patterns over time, making it possible to tell how long the fish has been napping and when it is about to wake up. Additional experiments under different light conditions showed that the sleep architecture is driven by the interplay between the internal biological clock and light exposure. Remarkably, all four sleep states and their architecture were also observed in two related Danio species, suggesting this sleep architecture is an ancestral feature.
The findings raise many questions, such as whether the eye movements serve a function or are a mere by-product of neural activity. “We are very curious about what roles the different sleep stages play,” said Jennifer M. Li, who together with Drew Robson co-leads the lab from which the study originated. “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.” The researchers are now taking a closer look at neural activity during night-time sleep to better understand the mechanisms and functions of the different sleep states.
Dr. Jennifer Mengbo Li
jennifer.li@tuebingen.mpg.de
Choudhary, V., Heller, C.R., Aimon, S. et al. Eye movement kinematics reveal novel circadian organization of sleep substates. Nat Commun 17, 4068 (2026). https://doi.org/10.1038/s41467-026-72222-0
A tracking microscope of the research group Systems Neuroscience & Neuroengineering at the Max Planc ...
Source: Jörg Abendroth
Copyright: Jörg Abendroth / MPI for Biological Cybernetics
Freely swimming larval zebrafish
Source: Jörg Abendroth
Copyright: Jörg Abendroth / MPI for Biological Cybernetics
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