Researchers led by teams at the University of California-Los Angeles and the University of Tel Aviv in Israel have found evidence that without sleep, some neurons in the medial temporal lobe—the part of the brain associated in part with recognizing different objects—have a harder time sending signals between each other. That may slow the reaction time of exhausted drivers.
“The very act of seeing the pedestrian slows down in the driver’s overtired brain,” Itzhak Fried, a neurologist and lead author of the paper, said in a statement. “It takes longer for his brain to register what he’s perceiving.”
The study, published (paywall) on Nov. 7 in Nature Medicine, relied on 12 volunteers in California who were receiving treatment for uncontrolled epilepsy. These participants—who were otherwise healthy adults—had eight to 12 electrodes implanted just below their skulls on the surface of their brains, designed to monitor the origin of their seizures. Doctors can stop seizures by locating where in the brain they start, and then removing that part.
It can sometimes be days between seizure episodes though. While the patients waited, they offered their time to UCLA researchers, who asked them to perform some simple tasks like classifying pictures as either human faces, animals, or places. When the brain interprets each of these objects, different neurons fire signals to one another. The researchers asked participants to do the same task on less and less sleep; at the end, four volunteers even stayed awake the entire night (which, coincidentally, is one way to trigger a seizure).
Because the electrodes were inside these people’s skulls, researchers could see each neuronal cell’s activity. When the participants were sleep-deprived, some of their neurons took longer to fire as they classified each picture. They also started emitting weaker electrical waves patterns, similar to those associated with sleep. This meant it was harder for these neurons to communicate messages, which translated into delayed reactions in the sleepy subjects.
Chiara Cirelli, a psychiatrist at the University of Wisconsin-Madison and co-author of the paper, published work earlier this year suggesting that while we sleep, our brains tidy up and organize the different connections between their cells. This process, she told Quartz earlier this year, is essential for our neurons and memory formations.
If we force ourselves to stay awake, this paper suggests, individual cells may try to sneak in catnaps. “Select regions of the patients’ brains were dozing, causing mental lapses,” Fried said.
This is why drivers who haven’t slept in a while are much more dangerous on the road, Fried told NPR. The neurons responsible for noticing pedestrians may duck out to get some rest and fail to alert drivers that they need to swerve.
To be sure, this was a small study; 12 people is normally not a large enough group to draw significant enough conclusions. But this was a fairly unique study: Normally, this type of research is far too invasive to conduct on otherwise healthy humans. Most studies looking at individual neurons are on other animals, and the majority of sleep studies on humans rely on external measures of brain activity. Fried thinks the data they got from tracking actual cell activity is evidence enough to start taking tired driving much more seriously. “Severe fatigue exerts a similar influence on the brain to drinking too much,” he said in a statement. “Yet no legal or medical standards exist for identifying overtired drivers on the road the same way we target drunk drivers.”