Whether periods of prolonged inactivity such as when sharks are resting on the seabed represent sleep or quiet wakefulness is unknown but the finding that it takes significantly higher levels of electrical stimulation to arouse inactive animals suggests that some species of shark do indeed sleep
Sleep is ubiquitous across the animal kingdom but despite anecdotal reports of sleep-like behaviour in nurse sharks and other seafloor-dwelling species, the question of whether sharks actually sleep has been intensely debated but remains unknown. A key criterion for separating sleep from other quiescent states is an increased arousal threshold. True sleep is characterised by a lack of movement that can be rapidly reversed and a decreased awareness of surroundings. These behaviours are regulated by an animal’s light-dark circadian rhythm and its homeostatic need to balance time asleep with time awake.
In an aquarium, researchers tested captive Port Jackson and draughtsboard sharks—species that have previously been shown to exhibit reversible periods of stillness and circadian activity patterns— by applying a series of mild electrical pulses to the sharks’ tanks to see how they responded. With both species, the researchers had to apply a stronger electrical pulse to get a response from an inactive shark than from one that was swimming.
Sharks deprived of rest, however, show no significant compensatory increase in restfulness during their normal active period following enforced swimming. Nonetheless, increased arousal thresholds in inactive animals suggest that these two species of shark sleep.