When sperm whales need a nap, they do what scientists call a drift-dive. They take up a straight, vertical position, drifting just beneath the surface of the water, usually in groups of five or six. Sperm whales fall into a deep, motionless sleep for 10-15 minutes at a time, a few times per day. They don't breathe while they sleep as they need to be conscious to take a breath. They spend just 7% of their day resting, getting less sleep than any other mammal in the world.