Do Sharks Sleep?
According to a recently released study, the GPA of first-year college students is greatly dependent on the amount of sleep they get each night (1). From a student perspective, most discourse is about sleep, or lack thereof. It is clear that sleep is integral to human success, and you might have seen some other animals sleep, like birds or your pet cat. What about sharks? Scientists have been asking this question for years and the answer is… probably?
The evidence of sharks sleeping is slim and definitely not conclusive for most species. Sleep can be defined with four criteria: “restfulness in a species-specific posture, often aligned more to one part of the 24-hr day, rapid reversibility to an awake condition, increased arousal threshold, and homeostatic regulation (2).” Out of the current 450 shark species or 1000 members of the Elasmobranchii taxon, only 33 sharks and one ray have been documented to do anything even resembling sleep, like being motionless(3). The nurse shark has displayed the most sleep criteria for a number of years, but a dedicated study of shark sleep hadn’t been performed until three years ago (2).
In 2020, it was found that at least two species of shark, the Port Jackson (Heterodontus portusjacksoni) and Draughtsboard (Cephaloscyllium isabellum), likely sleep based on behavioral analysis (2). They are both benthic sharks, which means they live in the benthic zone of the ocean, or the ocean floor, and displayed three out of four sleep criteria. This evidence suggested to the researchers that the studied sharks do sleep in short bursts similar to birds (4). Further study of the Draughtsboard shark in 2022 by the same researchers confirmed those sharks do in fact sleep, based on new metabolic data (5). So, at least one shark does sleep with data to back up the conclusion (2).
However, not all sharks inhabit the benthic zone. It is much harder to imagine a shark dozing off floating in the open ocean, especially while constantly swimming. Some sharks rely on movement to oxygenate their gills, so they are always moving forward to stay alive. This is much different from the bottom-dwelling sharks like the Port Jackson shark and Draughtsboard shark, which can get oxygen from opening and closing their mouths or buccal openings (6). Any true evidence of sleep in sharks that are ram ventilators (the sharks that have to swim constantly to survive) is entirely absent. Some of them perform some patterned circadian activities, but nothing that would suggest sleep as we currently categorize it (3).
A lot of research about shark sleep has been accidental and is recorded when studying other things about sharks. However, observation and a few completed studies suggest that buccal pumping allows sharks to sleep, and that ram ventilating sharks most likely sleep but in a way that we haven’t exactly pinpointed (7). It is unlikely that sharks secondarily lost the need to sleep because their environments are complex, and sharks exhibit at least moderate cognitive skills and learning. Some hypotheses that will be tested in the future are whether some sharks sleep unihemispherically (by only resting one half of their brain at a time), or whether ram ventilation sharks can replace constant swimming by placing themselves in a current that will move the water to their gills without effort from the shark (3). Further studies of shark sleep will be necessary to solve the unknowns and verify current conclusions.
- Creswell, J. et al. Nightly sleep duration predicts grade point average in the first year of college. PNAS. 2023. Web.
- Kelly, Michael. et al. Behavioural sleep in two species of buccal pumping sharks (Heterodontus portusjacksoni and Cephaloscyllium isabellum). Journal of Sleep Research. 2020. Web.
- Kelly, Michael. et al. Evidence for Sleep in Sharks and Rays: Behavioural, Physiological, and Evolutionary Considerations. Brain, Behavior, and Evolution. 2019. Web.
- The Secrets of Shark Sleep. Nature. 2022. Web
- Kelly, Michael. et al. Energy conservation characterizes sleep in sharks. The Royal Society. 2022. Web.
- Kelly, Michael. et al. Diverse Activity Rhythms in Sharks (Elasmobranchii). Journal of Biological Rhythms. 2020. Web.
- Tracey, Ferrier. Biologist puts shark sleep mystery to bed. AAP General News Wire. 2022. Web.
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