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.
“This period of silence and hyperpolarization of the cell membrane is probably related to the restorative function of sleep,” Cirelli told me. “The fact that there are these periods of total silence, that’s very typical and unique of sleep relative to wake and there might be something related to that.”
To understand the value of total neural silence, let’s look at another kind of sleeping animal—dolphins. Dolphins, along with whales, some sharks, and a variety of other underwater critters, need to stay moving to breathe. It follows that these animals can’t go completely unconscious like humans can—otherwise, the dolphins couldn’t come up for air, and oxygenated water would stop flowing over the sharks’ gills. And the research seems to bear that out: Brain scans show that dolphins never go into a full sleep state; instead, they turn off half of their brain for about eight hours a day, leaving the other half alert. This kind of rest has come to be called “unihemispheric sleeping.”
The closest humans ever get to unihemispheric sleep is when a person who’s extremely sleep-deprived shows signs of what Cirelli calls “local sleep in wake,” in which a few neurons turn off by themselves. The effect is unnoticeable from the outside, because the sleep-deprived subject is still awake and moving, but researchers are able to record the changes using deep-scanning technology that measures individual neurons.
But it’s not until we get access to real, deep sleep that we get a cognitive boost from rest. In other studies, test subjects who were made to identify letters flashed on a screen for several hundred milliseconds at a time generally did worse at the exam over the course of a day. Those who got to take a nap halfway through showed more cognitive recovery than those who simply rested quietly, suggesting that there’s a unique benefit to sleep that you don’t get with quiet wakefulness, microsleep, or unihemispheric sleep.