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Why Do Seals Sleep With Their Tails Out?

Some seals, such as the northern elephant seals, migrate annually between their foraging and breeding grounds. They may spend eight months at sea and the remainder on land, breeding, hunting and raising the pups.

Sleeping on Land
- Harbor seals rest by assuming a position with heads and tails pointed up in the air--the "banana-like" posture. With heads together in the center of a circle and tails pointed outward, they present a defensive position to predators. Seals fend off enemies by flipping their tails, which gains them time to turn and fight.
Sleeping at Sea
- Seals make repetitive deep resting dives that average 500 feet. They drop down into a head-first wobble like a falling leaf. The tail acts both as a braking mechanism to slow down the descent rate. This method allows them to sleep as they fall through the water, and reduces their work in returning to the surface to breathe.
Half-A-Brain Sleep
- Some seals sleep with only half of their brain at a time. This is called unihemispherical sleep. It allows them to be semi-alert for predators as they suspend themselves head-down vertically. The tail acts a reflex mechanism to control their fall. The seal rises at times to take a breath and then sink slowly back to the depths without an interruption of slumber.

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