Is Your Brain Sleeping While You're Awake?

If you think you can function on minimal sleep, here's a wake-up call: Parts of your brain may doze off even if you're totally awake, according to a new study in rats.

Scientists observed the electrical activity of brains in rats forced to stay up longer than usual. Problem-solving brain regions fell into a kind of "local sleep"—a condition likely in sleep-deprived humans too, the study authors say.

Surprisingly, when sections of the rats' brains entered these sleeplike states, "you couldn't tell that [the rats] are in any way in a different state of wakefulness," said study co-author Giulio Tononi, a neuroscientist at the University of Wisconsin, Madison.

Despite these periods of local sleep, overall brain activity—and the rats’ behaviors—suggested the animals were fully awake.

This phenomenon of local sleep is "not just an interesting observation of unknown significance," Tononi said. It "actually affects behavior—you make a mistake."

For example, when the scientists had the rats perform a challenging task—using their paws to reach sugar pellets—the sleep-deprived animals had trouble completing it.

(See "Secrets of Sleeping Soundly Uncovered.")

Sleep Allows Neurons to Reset?

Tononi and colleagues recorded the electrical activity of lab rats via electroencephalogram (EEG) sensors connected to the rodents' heads.
As predicted, when the rats were awake, their neurons—nerve cells that collect and transmit signals in the brain—fired frequently and irregularly.

When the animals slept, their neurons fired less often, usually in a regular up-and-down pattern that manifests on the EEG as a "slow wave." Called non-rapid eye movement, this sleep stage accounts for about 80 percent of all sleep in both rats and people.

The researchers used toys to distract the rats into staying awake for a few hours—normally "rats take lots of siestas," Tononi noted.

The team discovered that neurons in two sections of these overtired rats' cerebral cortexes entered a slow-wave stage that is essentially sleep.

Why Do We Sleep?

It's unknown why parts of an awake brain nod off, though it may have something to do with why mammals sleep—still an open question, said Tononi, whose study appears tomorrow in the journal Nature. (Read about mysteries of why we sleep in National Geographic magazine.)

According to one leading theory, since neurons are constantly "recording" new information, at some point the neurons need to "turn off" in order to reset themselves and prepare to learn again.

"If this hypothesis is correct, that means that at some point [if you're putting off sleep] you're beginning to overwhelm your neurons—you are reaching the limit of how much input they can get."

(See "Sleep Cherry-picks Memories, Boosts Cleverness.")

So the neurons "take the rest, even if they shouldn't"—and there's a price to pay in terms of making "stupid" errors, he said.

(See brain pictures.)

Even "Alert" People Make Mistakes

Sleep deprivation may have dangerous consequences, Tononi said—and those mistakes may become more common.

For one, many people are getting fewer z's. In 2008 about 29 percent of U.S. adults reported sleeping fewer than seven hours per night, and 50 to 70 million had chronic sleep and wakefulness disorders, according to the U.S. Centers for Disease Control and Prevention. Adults generally need about seven to nine hours of sleep a day, according to the National Sleep Foundation.

(Take National Geographic magazine's sleep quiz.)

What's more, you don't need to feel sleepy to screw up, Tononi emphasized.

"Even if you may feel that you're fit and fine and are holding up well," he said, "some parts of your brain may not [be] ... and those are the ones that make judgments and decisions."