© The Blowfish  Do Sharks Sleep? A great question! Especially considering many sharks have to keep moving to 'breathe'. So, we asked Tom to dig into the latest science to find out...

If you think dropping off is hard work at times, then spare a thought for the sharks and rays. It's one thing to suffer from insomnia. But another thing entirely to not even have science recognize your napping habits. That's where we currently stand. We simply don't know, or rather can't prove, if sharks sleep.

It's a huge and complex problem to study sleep. Even in humans, who are reasonably good patients, sleep research is slow and subject to a lot of interpretation and mystery. What we do know is the biological effects of sleep... lowering of the heart rate, flushing of the brain with cerebral fluid, and activation of may of the body's repair mechanisms. We can measure this via hard chemical or physical data. And, with terrestrial animals, we can look at immobility, reduced awareness, and relaxed posture to denote a sleeping state.

But all this is incredibly difficult to measure in sharks. Smaller, bottom-dwelling or sedentary species offer an opportunity for scientific study. So, some research has been done on their resting behaviours, but it's thin at best. Sleep studies on the larger, continuously swimming sharks simply doesn't exist. The best evidence we have is from recordings of shark behaviours where their activity stops, or significantly drops. A Bull Shark was captured drifting in an uncontrolled manner for 2 minutes. In which time it slowly sank as it glided along, before firing back up again and continuing on its way. It's easy to see why this might be considered sleep, albeit the speediest of power naps But equally the shark may have just been resting.

One thing we can be sure of is that sharks and rays must have some form of sleep. Be it a rapid rest or controlled shut down of parts of the brain. Like us, sharks have a pineal gland in their brain. This is the timekeeper of the body, responsible for our sleep cycles. We don't know exactly how this works in sharks. But, considering their place as evolutionary godfathers, it'd be a stretch to think sleep evolved independently of sharks.