How do Sharks Sleep

Sharks belong to a group of fish with cartilaginous skeletons. Sharks have existed for more than 400 million years. They range in size from a few centimeters, such as the dwarf lanternshark to sharks with a size of 12 meters (39 ft.), such as the whale shark. Sharks are found in all oceans around the world. Some species, including the great white shark, hammerhead shark and tiger shark are apex predators, meaning they are at the top of their food chain. Sharks usually possess five or seven gill slits which they use to get oxygen-rich flows of water, allowing them to breathe.

Habitat

Sharks are found inhabiting the seas all around the world. They typically do not live in fresh water, except for the bull shark and the river shark which can inhabit both fresh water and salt water. Sharks most commonly swim at depths of approximately 2,000 meters (7,000 ft.), and some may swim at even deeper habitats; however, sharks are hardly seen inhabiting profundities below 3,000 meters (10,000 ft.). The Portuguese dogfish holds the deepest report of a shark swimming at depths of up to 3,500 meters (12,000 ft.).

Feeding

Sharks are large marine predators. Shark species, including the hammerhead, tiger, blue and the great white shark are apex predators. In other words, they are at the top of their food chain without bigger animals feeding on them.  Despite its enormous size, the whale shark feeds only on squid, plankton and small fish. Basking sharks use ram feeding to obtain their food. Other sharks, including the whale shark use suction feeding. Other sharks, such as the cookie cutter shark possess enormous teeth which they use to slice flesh out of their prey. The great white shark and other large predators may either swallow their prey in whole or slice them in pieces with their huge sharp teeth.

Sleep

For a long time, it was thought that sharks had to remain in constant movement in order to breathe, and as a consequence of this, they were not able to sleep, but for a few minutes at a time. Some sharks can remain motionless at the bottom of the ocean while pumping water over their gills; however, with their eyes wide open, they follow the movement of other animals around them, including divers. While at the bottom of the sea, rich-oxygen water flows through their gills, allowing them to breathe. Like many fish, sharks obtain oxygen from ocean water through their gills; however, unlike other fish, the gills of a shark are not covered, but lie above their head.

Some species of sharks need to maintain constant movement; however, some sharks, such as the nurse shark possess spiracles which force water into their gills, allowing them to rest while maintaining a stationary position. Sharks do not sleep at all in the same way as humans do. On the contrary, they have periods of activity and inactivity. The anterior and posterior dorsal spiny spines coordinate swimming, allowing them to maintain movement while sleeping. It is theorized that sharks could possibly sleep in the same way as dolphins do., maintaining the cerebral consciousness of one cerebral hemisphere at a time, allowing them to rest while at the same time staying alert continuously. It is thought that sharks can remain restlessly with their eyes open, following the activity generated around them.

Sharks have been around for over 400 million years. Sharks have survived massive extinctions due to shark hunting and shark finning. The International Union for the Conservation of Nature has listed 50 species as vulnerable, from the more than 300 species assessed; however, only the great white sharks, the whale sharks and the basking sharks are protected internationally. According to the IUCN, sharks represent one of the greatest percentages of endangered marine species.