Reddit is a great forum for raising scientific questions, but the fact that it's discussion-based makes it difficult to know when a debate has settled on the best answer, objectively speaking. Exhibit A concerns the value of lying down with your eyes closed. How much does it do for you compared to actual sleep? The whole exercise can seem like a waste. Is it?
Part of what makes this question so slippery is that it hinges in large part on the matter of what sleep is actually for. We can all name the benefits of sleep, but saying what sleep accomplishes is a far cry from identifying what sleep is meant to do. The distinction is important. If the point of sleep is that being inactive frees up our energy for other tasks (say, recovering from a cold), we might expect lying in bed with our eyes closed—what some studies call "quiet wakefulness"—to accomplish much the same thing.
Researchers are growing increasingly confident, though, that sleep evolved specifically to recharge the brain. Dr. Chiara Cirelli, a neuroscientist at the University of Wisconsin, has been studying the difference between sleep and quiet wake in humans. She says that while we're awake, all of our neurons are constantly firing, but that when we're asleep, the neurons revert to an "up-and-down" state in which only some are active at a given time. During some stages of sleep, all neuron activity goes silent. And that's likely when the restful part of sleep takes place.