Sleeping With One Eye Open
Reading through Matthew Walker’s book Why We Sleep, I ran across this bit about animals who sleep with one half of their brain at a time. This allows one side to be asleep (with the eye on its side closed), leaving the other side to be awake (with its eye open). Dolphins are known to sleep this way. But birds do it with an interesting wrinkle, taken from the book:
In some species, many of the birds in a flock will sleep with both halves of the brain at the same time. How do they remain safe from threat? The answer is truly ingenious. The flock will first line up in a row. With the exception of the birds at each end of the line, the rest of the group will allow both halves of the brain to indulge in sleep. Those at the far left and right ends of the row aren’t so lucky. They will enter deep sleep with just one half of the brain (opposing in each), leaving the corresponding left and right eye of each bird wide open. In doing so, they provide full panoramic threat detection for the entire group, maximizing the total number of brain halves that can sleep within the flock.