Sharks have spent 450 million years perfecting survival. They can detect electrical fields measured in billionths of a volt, navigate entire ocean basins, and go months between meals. Yet one question has puzzled scientists for decades: do they sleep?
The short answer is yes, just not in any way that looks familiar to us.
What Does "Sleep" Even Mean for a Fish?
Sleep is not simply the absence of movement. Scientifically, it is defined by a set of criteria: reduced responsiveness to stimuli, a specific body posture, reversibility (the animal can be woken), and a compensatory rebound when sleep is denied. For years, sharks ticked enough of those boxes behaviourally, but no one had recorded the electrical brain activity to confirm it.
That changed in 2024, when researchers from La Trobe University and the University of Western Australia published a landmark study in the Journal of Experimental Zoology documenting the first electrophysiological correlate of sleep in a shark. Using draughtsboard sharks, a small, bottom-dwelling species found in New Zealand and southern Australian waters, the team recorded brainwave patterns during periods of behavioural rest that closely resemble what we see in sleeping vertebrates. The science is no longer circumstantial.
The Problem With Sharks That Cannot Stop Swimming
Here is where things get genuinely fascinating. Many shark species are obligate ram ventilators: they must keep swimming to pass oxygenated water over their gills. Stop moving, and they suffocate. So how does a great white shark sleep?
The most likely explanation, researchers believe, involves unihemispheric slow-wave sleep, a strategy where one half of the brain rests while the other stays active. Dolphins and some seabirds use this same trick to sleep while still surfacing for air or staying alert to predators. Evidence published in Brain, Behavior and Evolution suggests sharks may have developed comparable mechanisms, though confirming it in continuously swimming pelagic species remains one of the more difficult challenges in marine neuroscience.
For bottom-dwelling species like wobbegongs, nurse sharks, and Port Jackson sharks, rest is easier to observe. These sharks use buccal pumping, actively drawing water over their gills, so they can lie completely still on the seafloor for extended periods. Divers along Australia's east coast who have encountered a nurse shark motionless under a ledge at midday are almost certainly watching one sleep.
Energy Conservation Is the Whole Point
A 2022 paper in Biology Letters confirmed that sleep in sharks serves the same fundamental purpose it does in all animals: energy conservation. During rest periods, sharks showed measurably reduced metabolic activity. Given how much energy a large predator like a tiger shark or great white expends on long-distance migration, every calorie counts.
This also has a practical implication for ocean users. Sharks that are resting are less reactive, less likely to be moving toward surface activity, and in a fundamentally different behavioural state than when hunting. Understanding these patterns is part of the broader science of when and where sharks are most active, knowledge that directly informs smarter decisions about when to get in the water.
What We Still Don’t Know
Despite recent breakthroughs, shark sleep remains poorly understood for most of the approximately 540 known species. We have reasonable evidence for a handful of bottom-dwellers and some theoretical frameworks for continuous swimmers, but the sleep architecture of oceanic sharks, the species most likely to share water with surfers and divers, is still largely uncharted.
What we do know is this: sharks, like all animals, have biological rhythms that govern their rest and activity cycles. Research tracking diverse species found distinct activity patterns tied to time of day, reinforcing that sharks are not perpetually "on", they have quieter windows built into their biology.
The more we understand about those rhythms, the better equipped we are to share the ocean thoughtfully.
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