You may know this already, but some birds and sea mammals sleep with one eye open and the other closed. During this form of sleep, called “unihemispheric slow-wave sleep” (USWS), half of the brain (the one opposite the closed eye) gets a rest while the other half remains on duty, keeping the animals safe from predators or, if they’re marine, able to keep surfacing to breathe.
I’ve just learned of a ten-year-old study in Nature by Neils Rattenborg et al. looking at USWS in mallards, who often sleep in rows. The researchers found that birds sleeping at the end of a row engaged in USWS 31% of the time, as opposed to only 12% for ducks in the middle. Moreover, in “edge” birds, the eye facing away from the center of the group was open 86% of the time, as opposed to only 52% — not different from random — for “inside” birds. EEG recordings showed that this eye-closing indicated sleep on the opposite side of the brain.
What is even more amazing is a fact documented in this wonderful Radiolab program on sleep rebroadcast yesterday (do listen to it if you have a free hour): the “edge” ducks occasionally turn themselves around 180 degrees. When they do this, the new outer eye is the one that remains open.
This behavior apparently allows both sides of the brain get some sleep! (We still don’t know why sleep is biologically essential, but it is: humans and rats deprived of sleep, for instance, eventually die.)
Another cute anecdote: Rattenborg also reports that he once saw a pet cockatiel sleeping next to a mirror: “The mirror-side eye closed as if the reflection were a pal, and the other eye stayed open.”
The biological imperative of sleep remains one of the great mysteries of science.
Rattenborg, N. C., S. L. Lima, and C. J. Amlander. 1999. Half-awake to the risk of predation. Nature 397:397-398.