It has long been believed that sharks do not slumber at all; the notion that certain sharks must remain active in order to breathe has led to this belief.
Nevertheless, a recent study verifies what circumstantial evidence and previous studies have long suggested: these animals, like humans, do sleep.
Although two distinct stages of sleep are well understood in birds, mammals, or even octopuses – implying that each cycle plays a critical function in human physiology – nothing is documented about this mechanism in cold-blooded backboned species. As a result, the scientists looked for indicators of slumber in the draughtsboard shark, species they had previously determined to be nocturnal.
In an earlier study, the experts proved that it takes more electromagnetic stimulation to elicit a response from a shark when it seems to be sleeping — but they could not establish that this resting condition was really sleep.
Observing the sharks for 24 hours indicated a constant fall in oxygen levels throughout these moments of restfulness, showing that those lasting longer than 5 minutes are truly asleep.
Sharks shut their eyes more often throughout the day, implying that eyelid closure is more probably caused by external stimuli such as the presence of sunlight than by the sleep condition itself. 38% of sharks maintained their eyes open throughout the night, despite the fact that other evidence indicated they were deep sleeping.
The scientists discovered that a sleeping shark’s posture is a more reliable indication. Draughtsboard sharks maintained their bodies flat while sleeping.
A 1970s research discovered that the processes that control swimming motions in the little spiny dogfish shark species are situated in the spinal cord, not really the brain, suggesting that sharks may be able to continue moving while unconscious.
There is still a great deal about sleep that is unknown, and knowing how this phenomenon unfolds in sharks may give insight into how our own capacity to sleep originated.
This study was published in the journal Biology Letters.
