"This period of silence and hyper-polarization 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 brains 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.

"This function is only happening when there is a real nap with real sleep as measured with EEG," said Cirelli.

Lying down isn't completely useless—it does help your muscles and other organs relax. But you'd get the same results just from reclining on the couch. So sleep is still your best friend.

The useful takeaway is that your best move, if you've been in bed for 20 minutes and still aren't dozing off, is to get up and engage in a low-light, low-stress activity like reading until you begin to feel tired. Taking your mind off of "Why am I not sleeping?! I need to sleep!" is crucial. When you do get up, though, don't use your computer or phone or watch TV—the blue-colored light from the screens tricks your body into thinking it's daytime and not releasing melatonin. Sweet, sweet melatonin.

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