Dolphins must surface periodically to breathe air; many can only hold their breath for seven minutes. During sleep, dolphins shut down one hemisphere of the brain while the other keeps an eye out for trouble.

Originally released by Smithsonian Media, 2014. Streaming video file encoded with permission for digital streaming by Infobase on March 10, 2015.

How Do Dolphins Sleep Without Drowning?: Ask Smithsonian (1:00); .