When people don't get enough sleep, certain brain cells literally slow down.
A study that recorded directly from neurons in the brains of 12 people found that sleep deprivation causes the bursts of electrical activity that brain cells use to communicate to become slower and weaker, a team reports online Monday in Nature Medicine.
The finding could help explain why a lack of sleep impairs a range of mental functions, says Dr. Itzhak Fried, an author of the study and a professor of neurosurgery at the University of California, Los Angeles.
"You can imagine driving a car and suddenly somebody jumps in front of the car at night," Fried says. "If you are sleep-deprived, your cells are going to react in a different way than in your normal state."
The finding comes from an unusual study of patients being evaluated for surgery to correct severe epilepsy.
As part of the evaluation, doctors place wires in the brain to find out where a patient's seizures are starting. That allows Fried and a team of scientists to monitor hundreds of individual brain cells, often for days.
And because patients with epilepsy are frequently kept awake in order to provoke a seizure, the scientists had an ideal way to study the effects of sleep deprivation.
In the study, all the patients agreed to categorize images of faces, places and animals. Each image caused cells in areas of the brain involved in perception to produce distinctive patterns of electrical activity.
"These are the very neurons [that] are responsible for the way you process the world in front of you," Fried says.
Then, four of the patients stayed up all night before looking at more images.
And in these patients, "the neurons are responding slower," Fried says. "The responses are diminished, and they are smeared over longer periods of time."
These changes impair the cells' ability to communicate, Fried says. And that leads to mental lapses that can affect not only perception but memory.
The team also found evidence that sleep deprivation affects some areas of the brain more than others. It was as if certain regions of the brain were sleeping, while others remained vigilant, Fried says.
The research adds to the evidence showing it's important to avoid driving when you're sleepy, Fried says.
Drowsy driving in the U.S. is responsible for more than 70,000 crashes a year according to the Centers for Disease Control and Prevention, based on estimates and statistics gathered by the National Highway Traffic Safety Administration.
Fried says his team's finding also supports efforts to limit the hours worked by doctors in training, noting that he worked very long hours as a neurosurgery resident.
Now, he says, "I am trying to impose the lesson I learned from my research on myself."
KELLY MCEVERS, HOST:
A night without sleep leaves most of us feeling a little flat or sluggish. Now scientists have found a biological explanation. NPR's Jon Hamilton reports on evidence that when brain cells are deprived of sleep, they slow down.
JON HAMILTON, BYLINE: When you see something unexpected, your brain tries to respond quickly.
ITZHAK FRIED: You can imagine yourself driving a car and suddenly somebody jumps in front of the car at night.
HAMILTON: Itzhak Fried is a brain researcher at the University of California, Los Angeles. He says there's lots of evidence that sleepy drivers take longer to hit the brakes. But Fried wanted to know why, so he and a team of researchers studied how certain brain cells respond to information coming from the eyes.
FRIED: We saw the very neurons, you know, which are responsible for the way that you process the world in front of you on the highway or on the job or in any situation.
HAMILTON: The team was able to study these neurons in a dozen people with severe epilepsy. These people often spend a week or more with tiny wires in their brains to help doctors figure out the source of their seizures. And this allowed the scientists to monitor the bursts of electrical activity produced by specific brain cells as patients looked at images of faces, places and animals. Then, Fried says, four of the participants were kept awake all night and given the image test again. This time, Fried says, there was a clear difference.
FRIED: The neurons, the nerve cells, are responding slower, the responses are diminished, and they are smeared over longer periods of time.
HAMILTON: In other words, each cell took longer to produce the electrical bursts it uses to communicate with other cells and the signal was weaker. What's more, the brain cells tended to show this sluggish behavior just before a participant had a mental lapse while trying to classify an image. The finding appears in this week's edition of the journal Nature Medicine, and it's made Fried think differently about his own sleepless nights. He says he's a bit horrified by the long shifts he spent treating patients during his training as a neurosurgeon.
FRIED: I remember with some horror, you know, the kind of hours that we were subject to.
HAMILTON: Since then, training programs have imposed limits on the hours doctors can work. Fried says that's probably a good thing for patients. He also says he's working to get more rest than he used to.
FRIED: I'm trying to impose the lesson that I learned from my research on myself as well.
HAMILTON: One night at a time. Jon Hamilton, NPR News.
(SOUNDBITE OF ISAAC HAYES SONG, "WALK ON BY") Transcript provided by NPR, Copyright NPR.