If you have ever come across apparently slumbering whitetips or nurse sharks on a reef dive, you might be surprised to learn that there is a popular belief that sharks never sleep.
This appears to have come about mainly because of those sharks such as great whites and makos that have to keep moving to breathe.
Relatively little research has been carried out into sleep in cold-blooded vertebrates, but now an Australian study offers what scientists say is the first physiological evidence that sharks do sleep.
Across a 24-hour period, University of Western Australia ecophysiologist Michael Kelly and his team compared metabolic rates in draughtsboard sharks (Cephaloscyllium isabellum) that were displaying external signs of sleeping with the rates of actively swimming sharks.
The draughtsboard is a carpet shark endemic to New Zealand, named for its pattern of blotches and known to be nocturnal. It can remain motionless for long periods because its facial muscles keep pumping oxygenated water across its gills – unlike those sharks that rely on ram-ventilation through constant swimming.
The researchers found that the sharks’ oxygen levels decreased consistently during their resting periods, and were able to confirm that when these levels extended past five minutes the sharks were indeed asleep.
The team also investigated the sort of behaviour that typically indicates sleep in other animals (including humans) such as closed eyes and “postural recumbency” – lying down – in an attempt to establish relationships between the sharks’ physiology and their behaviour.
They found that a lower metabolic rate and a flat posture were good indicators of sleep in draughtsboard sharks, but that closed eyes were not necessarily any indication that a shark was having a nap.
The draughtsboards were more likely to close their eyes when sleeping during the day, so this appeared to be no more than a reflex action to keep the light out. In darkness 38% of sharks kept their eyes open even when other factors indicated that they were sleeping.
The sharks’ periods of sleep were necessary for their energy conservation, the team concluded, while conceding that how sharks that use ram-ventilation to breathe manage to achieve the same end remains a mystery. One theory is that they effectively swim on autopilot, using mechanisms in the spinal cord rather than the brain.
The researchers hope to be able to study brain activity in sleeping sharks in future to learn more about the process. Their study is published in the journal Biology Letters.