India, Nov. 2 -- Sleeping with one eye open may sound like something out of a science-fiction film, but it is a real biological adaptation found in several animal species. Dolphins, mallard ducks and fur seals are among the animals that can rest while remaining partly alert to their surroundings.

This ability is known as unihemispheric slow-wave sleep (USWS). Unlike humans, who generally sleep with both brain hemispheres resting together, animals that use this method allow one half of the brain to enter slow-wave sleep while the other remains relatively alert.

The eye connected to the resting hemisphere closes, while the opposite eye can remain open. After a period of time, the animals can switch sides, allowing both halves of the brain...