Whales and dolphins must come to the surface of the water in order to breath or they will drown. Unlike humans on land who breath automatically when asleep, whales must make the conscious decision to breath; so how do whales get any sleep? Evidence and studies indicate that whales allow one half of their brains to sleep, while the other half stays alert to continue breathing and looking out for danger, usually with only one eye open. This type of sleep is known as unihemispheric sleep, as only one brain hemisphere sleeps at a time. Whales and dolphins periodically alternate which side is sleeping so they can get the required amount of rest.

When sleeping, whales typically stay in a horizontal or vertical position close to the surface of the water. Living in a water environment means whales and dolphins can lose a great deal of body heat when not active or moving, so some species keep swimming even while they are sleeping. Humpback Whales have been found resting motionless on the surface of the water, but for only 30 minutes so they can start moving again to keep warm.

Dalls Porpoise sleeping horizontally

The ability of different species to hold their breath varies between a few minutes to over an hour, so their sleeping habits vary. Sperm Whales spend just 7 percent of their day napping for a period of 10 to 15 minutes each time. Bottlenose Dolphins and Belugas can spend more than 30 percent of their day sleeping, and Gray Whales spend up to 41 percent of their time resting or sleeping. Dolphins generally sleep at night but only for a couple hours at a time. Like whales, Dolphins, and the related Dall’s Porpoise, sleep with one eye open to allow one-half of the brain to sleep. They generally sleep along the surface of the water but occasionally sleep along the bottom of a shallow area and rise to the surface for air as needed.

From a distance, sleeping whales and dolphins can look like logs in the water; boaters should take care to keep their distance from whales and other marine mammals that are at rest, as well as when these animals are underway.

Photo by Scientific American
Photo by Alchetron Encyclopedia