Bees, birds and iguanas do it, but no one is sure why animals sleep. “Every animal studied engages in some form of sleep or sleeplike behavior,” says Steven L. Lima, a biology professor at Indiana State University and an animal sleep expert. Lima and most researchers believe sleep “has some sort of critical maintenance or restorative effect on neural tissue.” But the unconscious state has a cost: it makes animals vulnerable to predators. Lima has found that in some birds, therefore, only half the brain rests at once. The other half stays alert, and the eye it controls stays open. The list includes pigeons, ducks, domestic chickens and a few other birds.

Most mammals cannot pull off the trick (dolphins can), Lima says, yet “humans are frequently subjected to situations—combat, travel and other stressful environments—where they need to decide when and how much to sleep.” The best we can do is cater to our sleep “architecture,” he explains. “The first two to three hours of deep sleep seem to be the most vital,” Lima says, “while we can do without much of the REM [rapid eye movement], or dream, sleep that comes later in the night—at least over the short term.”