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