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."