Humans and other mammals exhibit diverse sleep phases. One of these is readily identifiable by rapid eye movements occurring beneath closed eyelids, hence its designation: REM sleep (Rapid Eye Movement sleep). It is during this particular state that we experience our most vivid and intense dreams.
The question of whether creatures beyond mammals experience analogous distinct sleep states—and, more specifically, sleep characterized by eye movements—has long been a matter of debate. Researchers at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, have, for the first time, uncovered a complex sleep architecture within fish. Zebrafish (Danio rerio), like most bony fish, are known to go through periods of inactivity lasting several minutes, during which they become less responsive to external stimuli. The current investigation demonstrates that these extended repose periods are not uniform; the research group identified three distinct sleep types, each associated with its own characteristic pattern of eye movement, alongside a fourth type exhibiting a complete absence of eye motion.
Past studies have noted intermittent eye movements in fish during brief immobility, but no one had systematically documented these characteristics during natural sleep. “So I was amazed when I first saw the fish eyes moving in this peculiar way,” remarks Vikash Chaudhary, who co-authored the study with Charles Heller. Chaudhary suggests that the success might be attributed to a novel experimental setup: “Our methodology is unique because we were the first to simultaneously record eye and body movements over a full 24-hour cycle in freely swimming fish.” This was made feasible by a specialized tracking microscope capable of monitoring zebrafish larvae as they moved unimpeded. Crucially, larval zebrafish possess transparent brains, enabling the microscope to capture whole-brain neural activity in real-time.
By correlating brain activity recordings with behavioral observations, the team ascertained that each state adheres to its own circadian rhythm. During the nighttime, sleep lacking discernible eye activity predominates. Of the three quiescent states that involve eye movement (QEM), one peaks at night, while another intensifies toward the morning. Most surprisingly, QEM-1, the most frequently observed QEM state, occurs almost exclusively during the day. This is particularly unexpected given that fish in QEM-1 are exceptionally difficult to rouse, leaving them susceptible to predators.
During this daytime sleep phase, the team also observed a significant overall reduction in brain activity. Combined with other neurological and behavioral indicators, this confirms that QEM-1 truly represents a sleep condition analogous to a daytime nap. The brain activity recordings also revealed predictable temporal patterns, allowing the determination of how long the fish had been dozing and when it was poised to awaken. Further experimentation under varied lighting conditions established that the sleep architecture is governed by the interplay between the intrinsic biological clock and light exposure. Significantly, all four sleep states and their respective architecture were also present in two related zebrafish species, indicating that this sleep structure is an inherited trait.
These findings prompt numerous inquiries, such as whether eye movements serve a functional purpose or are merely a byproduct of underlying neural processes. “We are very curious about what roles the different sleep stages perform,” stated Jennifer M. Lee, who co-leads the laboratory where this research took place with Drew Robson. “Sleep is vital for numerous processes, ranging from memory consolidation to waste removal, but we still don’t fully grasp why and how it is time-organized. Zebrafish, with their transparent brains, offer us a powerful instrument to unravel this mystery.” Currently, the researchers are scrutinizing the neural activity during nighttime sleep to obtain a deeper understanding of the mechanisms and functions underpinning the various sleep stages.
About the Author
