Myth Busted: Sharks DO Sleep!
Updated: Apr 27, 2021
Sleeping is wide-spread in the animal kingdom, from hibernating bears to micro-napping bullfrogs. Even insects, worms and jellyfish sleep! The changes in brain state associated with sleep are critical for healing, cellular repair, processing and forming memories. Yet, studies assessing the role of sleep and its mechanisms in fish are quite sparse.
As it is challenging to record brain wave activity in water, studying sleep in fish, like sharks, has been limited in the past. Whatsmore, the elusive nature of sharks and their living in an environment that is challenging for human beings to sample has hindered research. What we do know about sleep in sharks has been deduced from the study of only a small number of individuals or from anecdotal evidence, but it does appear that sharks do sleep!
Many people assume sharks cannot sleep, due to the pervasive myth that if a shark stops swimming it will die. This idea comes from overgeneralisation and poor knowledge of the shark respiratory physiology. The spiny dogfish (Squalus acanthias), does indeed, never stop swimming, but many sharks so not necessarily need to keep swimming to "ventilate" (passing oxygenated water over the gills so that the shark can 'breathe').
There are two main types of ventilation, known as "buccal pumping" and "ram ventilation". Buccal pumping involves raising and lowering the floor of the mouth (aka the "buccal cavity") to draw oxygenated water into the mouth and force it over the gills. This method is common in the "Heterodontiformes" (the bullhead sharks). On the other hand, ram ventilation is common in the more active, pelagic species of shark, such as the "Carcharhiniformes" (like the hammerheads). These sharks push oxygenated water into the mouth and over the gills through motion, meaning they must be moving forwards to ventilate.
However, many ram ventilating species, such as the "Lamniformes" (like the great white shark), "Squaliformes" (dogfish) and "Carcharhiniformes" are also capable of brief periods of inactivity in which they can ventilate by buccal pumping and sedentary species ram ventilate when they are swimming. So it really is not as cut-and-dry as "if they stop they die" (Kelly et al, 2019).
There is evidence that both buccal-pumping and ram ventilating sharks species do sleep, or at least rest! However, this sleep is not necessarily like that which humans experience and can vary enormously between species.
Buccal Pumping Sleep
Sharks that ventilate via buccal pumping are known to have extended periods of rest on the substrate, where they can be quite still. For instance, the greater spotted catshark (Scyliorhinus stellaris) appear to sleep by half-closing their eyes and resting their head against a rock (Kelly et al, 2019).
Similarly, nurse sharks (Ginglymostoma cirratum) spend significant periods resting in caves, during which they do not react to divers. This species is "nocturnal" (meaning they rest during the day and are active at night. Several other species of shark follow the same daily pattern (known as a "circadian rhythm"), including the swell shark (Cephaloscyllium ventriosum) and the horn shark (Heterodontus francisci).
There are also many species that rest during the day and then become active during dawn and/or dusk (known as "crepuscular"). Such as the Port Jackson shark (Heterdontus portjacksoni), the lemon shark (Negaprion brevirostris) and and whitetip reef shark (Triaenodon obesus), to name a few (Kelly et al, 2019).
Ram Ventilating Resting
There are several ram ventilating sharks with a defined circadian pattern. For example, the bull shark (Carcharhinus leucas) and the blue shark (Prionace glauca) rest in caves for extended periods and only respond when they are handled very roughly. Similarly, basking sharks (Cetorhinus maximus) bask in the sun near the surface during the day and are not disturbed even when they are touched. Scalloped hammerhead sharks (Sphyrna lewini) gather in groups around a seamount, and have very low levels of activity during the day. They will not even respond if suitable prey is around, but are aroused at night to actively hunt.
However, many species of shark which predominently ram ventilate have a less clearly defined circadian rhythm compared to those that ventilate via buccal pumping. This suggests they either sleep in short bursts or, if they sleep over extended periods they continue swimming throughout.
Many marine mammals sleep whilst swimming by only using one half of their brain at a time, whilst the other half of the brain sleeps. This is known as "unihemispherical sleeping". It is possible that sharks sleep in a similar way.
It is also possible that sharks may sleep by resting certain regions of the brain one at a times. Researchers speculate that sharks may shut down the "telencephalon" (the region of the brain involved in relatively advanced cognitive tasks, like problem solving) and retain only a few 'awake' pathways which control the tail, allowing them to continue swimming whilst resting.
It has also been suggested that sharks may sleep with their whole brain, but switch to a passive method of ventilation, in order to avoid the need to keep swimming. This would be possible using water currents; resting a region with a strong current would allow oxygenated water to continue to flow over the gills. There are several anecdotes from divers, of groups of inactive sharks resting in strong currents (Kelly et al, 2019).
We will need to develop a method which allows us to monitor brain waves in order to understand sharks' sleep better... Who knows, maybe they even dream!
References
Kelly ML, Collin SP, Hemmia JM & Lesku JA (2019). Evidence for Sleep in Sharks and Rays: Behavioural, Physiological, and Evolutionary Considerations. Brain Behaviour and Evolution, 94, 37–50. Access online.
By Sophie A. Maycock for SharkSpeak.