Dolphins sleep with only half of their brain at a time.

Dolphins are voluntary breathers. This means they must be conscious to swim to the surface and take in air. If they fell into a deep sleep like humans do, they would stop breathing and drown.Research by the National Marine Mammal Foundation shows dolphins can stay alert for 15 days straight. They do this by switching which side of the brain is resting. When the right side of the brain sleeps, the left eye stays open and connected to the awake left side of the brain.In 2012, Dr. Brian Branstetter led a study on dolphin alertness. Dolphins had to find targets using sound for five days without a break. Their accuracy stayed perfect the entire time. This proved that the resting half of the brain does not slow down the active half.Tests show that each side of a bottlenose dolphin's brain sleeps for about four hours a day. While resting, the active side of the brain keeps the dolphin's muscles moving. This allows them to keep swimming slowly near the surface. Scientists call this behavior logging because the dolphin looks like a floating log.