It has been said that sharks don't sleep at all, and that some sharks must stay on the move to facilitate their breathing.
A new study confirms what anecdotal evidence and other research has been saying for a long time.
We have provided the first evidence of sleep in sharks, according to the team, led by Michael Kelly from the University of Western Australia.
It's well known that birds and mammals have two sleep phases, but little is known about the process in cold-blooded animals.
The team investigated the signs of sleep in the draughtsboard shark, which they had previously discovered were nocturnal animals.
The researchers did not confirm that the resting state of the shark was sleep.
Monitoring the sharks over a 24 hour period revealed that their oxygen levels decreased during rest periods.
Kelly and team explain that sleeping sharks have reduced responsiveness to stimulation.
The sharks closed their eyes while asleep more often during the day, suggesting that the presence of light is more important than the sleep state. 38 percent of sharks kept their eyes open, even though other indicators suggested they were not awake.
The team found that posture is a better indicator of a sleeping shark. The sharks kept their bodies flat while snoozing.
The shark is able to remain motionless for extended periods of time thanks to their buccal pumps which keep oxygenated water flowing across their gills.
The great white shark, which has a pump and uses forward swimming to push oxygenated water into their mouth and over their gills, is not one of the species that have this pump. This is called ram-ventilation.
If sleep is common across all sharks, how would the swimmers achieve it while still on the move?
The way these sharks control their swimming motion is suspected to be the reason for it. A study in the 1970s found that the mechanisms that control swimming movements in the small spiny dogfish shark are located in the animal's spine and not in the brain.
Understanding how sharks sleep could give us clues on how our own sleep evolved.
Future research should focus on other indicators of sleep, such as changes in brain activity, for a more complete portrait of sleep in these vertebrates.
This research was published in a journal.
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