A common myth about sharks is dispelled

For ages, people have believed the widely circulated myth that sharks cannot sleep. The theory was that they were unable to stop moving because resting on the bottom of the ocean would compress their abdomens, making them unable to circulate water through their gills, effectively starving them of oxygen.

Sharks that are in motion are able to maintain an effortless flow of water over their gills. Breathing is more laborious for a stationary shark, which supported the misconception that they would be unable to breath if they stopped to sleep.

The truth is that some species of shark are actually capable of resting while they are in motion, with parts of their brains being temporarily shut down. Their brainwaves are altered and they are only partially aware of their surroundings. It is even believed that they shut down different parts of their brains sequentially.

Some sharks also make use of areas that have current so that they can sleep with their mouths open to allow water to continually flow over their gills.

But what has surprised scientists in recent years is that some species of shark such as this Nurse Shark are able to lie completely motionless in an area devoid of water current, yet they are still able to breath. This Nurse Shark was found in a circular recessed area of coral approximately 15 feet in diameter and 6 feet deep. The water was 20 feet deep in this area and the current was negligible.

The shark remained motionless from the time that it was first observed by scuba divers entering the water to the time that their dive was complete, one hour later. The shark was aware of the diver recording its slumber and it chose to move on from its resting spot.

Scientists are now in agreement that not all sharks must remain in motion and that sharks do indeed sleep.