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you ever been so exhausted that you feel basically half-asleep? Turns out, that may be what’s literally going on in your brain, according to a new study published in Nature.

In a study of rats, researchers found that when the animals were deprived of sleep, parts of their brains (specifically, neurons in two areas of the cerebral cortex) switched into a sleeplike state, even while the rest of the brain stayed awake — and the rats themselves appeared completely alert.

The new findings may help researchers better understand how sleep-deprivation affects human performance and may even shed light on why sleep is necessary to begin with.

Researchers led by Giulio Tononi of the University of Wisconsin-Madison measured the electrical activity in the brains of 11 rats, which they kept awake past their bedtimes by continuously introducing new objects into their cages. They found that the activity in some brain areas in the rats showed brief descents into “slow wave” sleep patterns. That’s the type of sleep we experience for 80% of the night, the kind that mostly doesn’t involve dreaming.

In other words, although the rats were awake, some of their brain cells were not. And the longer the rats were forced to stay awake, the more “sleepy” regions their brains registered.

The scientists trained eight rats to reach for difficult-to-grab sugar pellets while they were sleep-deprived. Rats that had brain regions of the frontal cortex that were “offline” showed diminished performance: they were 37.5% less likely to grab the pellet successfully than rats with awake brains.

It hasn’t been shown yet that the same phenomenon occurs in human brains, but the findings do shed light on why sleep-deprivation is associated with impaired performance. (It also helps explain in part why torturers and cult leaders like to keep their victims from sleeping; in general, judgment, ability to resist coercion and willpower decline as the need for rest rises.) People who have been sleep deprived for 24 hours, for instance, show the same driving-skills handicaps as drunk people — indeed, sleepy driving is second to drunk driving in causing the greatest number of fatal crashes.

The researchers write, “These initial findings raise the intriguing possibility that ‘local sleep’ in an awake brain may be responsible for cognitive impairments due to sleep deprivation or restriction.”

So is there a threshold for how much sleep a person needs before crossing into total impairment? NPR’s Ira Flatow put that question to Christopher Colwell, a psychiatry professor at UCLA Medical School, who said:

There is a range. … Most people, though, do need about eight hours of sleep. And this can actually be documented especially in vigilance tasks. Humans, if they’re not getting enough sleep, really perform poorly on vigilance tasks. So think of those air-traffic controllers, for example.

As you’re moving down to six hours, performance decreases on those tasks. … If you move all the way down to, like, four hours of sleep per night, then you start getting a number of neural endocrine changes in your body. So, basically, your stress levels go really high, your performance goes down, and you really get serious physiological changes with four hours [of sleep].

But most people aren’t at four hours. Most people are more like at the six-hour stage, where you see some decrease in performance but generally are getting through our days fine.

You may be making it through the day, but if you feel half-asleep doing it, you probably ar

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