Half-asleep rats look wide awake

Parts of the brain can doze off while an animal remains active

Some parts of a rat’s brain can fall asleep even while the animal plays and seems wide awake, a new study shows.

Researchers at the University of Wisconsin–Madison and their colleagues in Italy kept rats up four hours past the rodents’ usual bedtime. Even though the rats stayed awake, electrodes implanted in their brains showed that some brain cells went to sleep while neighboring ones remained active, the team reports in the April 28 Nature.

Rats with sleeping neurons were also prone to making mistakes during slightly difficult tasks, a finding that may have implications for sleep-deprived people.

“And it would be very insidious because nobody would be able to tell there was anything wrong with you,” says Giulio Tononi, the University of Wisconsin neuroscientist who led the work. The rats in the study weren’t “staring off into the void or anything,” he says.

The findings add to a growing body of evidence that sleep doesn’t necessarily involve the whole brain at one time. Many scientists previously thought that a central control center determined when the brain sleeps and wakes. But researchers have been building a case for the past two decades that sleep may originate in single cells and eventually spread all over the brain.

As far as the researchers could tell, the rats were fully awake and playing with objects the researchers had supplied to keep the animals up past their bedtimes. Only the electrodes implanted in two parts of the brain recorded the neuron naps.

But just because the rats weren’t nodding off doesn’t mean their brains were working well. The team tested the sleep deprived rats’ ability to reach through a plexiglass wall and grasp a sugar cube. The task involves some coordinated moves, such as rotating the wrist, that aren’t part of a rat’s normal repertoire, so if the animals’ brains aren’t firing on all cylinders, the grab could fail.

When brain cells in the motor cortex — a part of the brain that controls movement — fell asleep, rats failed in attempts to grab the sugar cubes for several hundred milliseconds afterward. But sleeping neurons in the parietal cortex, which is not involved in the task, did not lead to mistakes.

If the finding applies to people it could mean that lost sleep is even more dangerous than previously believed, leading to slips of the tongue, driving mistakes, errors of judgment or other problems.

“So many humans are walking around with a sleep debt,” says Christopher Colwell, a neuroscientist at UCLA. “This is probably part of the everyday situation for a lot of people.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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