“Do sharks sleep?”

This is a question that many-a-curious-George has asked him/herself for years. But the answer, unfortunately, remains elusive.

For long, science has told us that sharks need to constantly swim to stay alive. Stop swimming and they die. Unfortunately, sharks and their sleeping habits are one of the least-studied aspects of marine biology. Add to this the immense behavioural diversity that each of the 400 species of shark in the world exhibit and you have yourself a recipe for confusion.

A breath of truth

There are basically two categories of sharks you need to consider when studying shark sleeping behaviours:

• Sharks that have spiracles
• Sharks that don’t have spiracles

Spiracles are two small openings located on each side of the shark’s nose, just behind its eyes. They are actually a type of gill-slit that are designed to let water pass through the shark’s body when the shark is resting on the ocean floor. These spiracles work even when the shark is covered in sand. Only certain forms of bottom-dwelling sharks like rays, nurse sharks, wobbegong/carpet sharks and skates have spiracles.

When a shark has spiracles, it can easily rest on the floor without having to worry about breathing. The spiracles push the water into the shark’s body and enable the processing of oxygen. So, instead of being forced to be on-the-move always, sharks with spiracles can rest on the ocean floor and do what they do best, ambush their prey.

So, what about sharks without spiracles?

See, this is where things get really tricky. Scans of “sleeping” sharks indicate that while the brains are inactive and unconscious, the rest of the shark is active and working. Just like in many other animals, it’s the spinal cord that is responsible for the swimming motion in sharks. Research shows that the synapses and neurons in a shark’s spinal cord are always active and always engaged in exchange of neural information, irrespective of what the rest of the body does.

What this means is that, the parts of the shark’s body responsible for swimming never stop working, even if the rest of the shark is asleep. So, the question of sleep & breathing doesn’t arise here, since sharks without spiracles don’t engage in what we humans traditionally term as “sleep”. Their brains remain unconscious, while their bodies continue to move.

But do sharks really sleep?

This still doesn’t answer the basic question – spiracle or no spiracle, do sharks actually sleep?

The answer – not really; at least not according to our description of “sleeping”.

Take a look at any dictionary and you’ll see that “sleep” is defined as an activity where
“the mind and the body are suspended of consciousness“, where they “remain inactive until exposed to external stimuli“.

Based on observable evidence, sharks don’t really sleep. Often, bottom dwellers remain stationary, while being completely mentally active, observing the movement of animals swimming past. Irrespective of whether they ambush their prey or not, these sharks remain awake at all times.

Then there are sharks who do exhibit sleep-like behaviours, but don’t fall into deep slumber like we (or other animals) do; continuing to move through the water, always.

Why exactly these sharks remain partially-unconscious or lay so still, is unknown. But one thing is certain; these behaviours definitely aren’t proof of sharks resting or sleeping.

Of course, with the question of sleep, comes the question of dreams. Do sharks dream? The answer – maybe not. Since they don’t engage in traditional sleeping patters, scientists still aren’t certain whether they engage in REM and non-REM cycles; making any question related to dreaming redundant until further evidence is available.

For now, sharks and their sleeping habits remain heavily-shrouded in mystery. Let’s hope the future helps us swim past these cloudy waters towards clearer explanations.

-NISHA PRAKASH

(P.S: Featured image)