A new study published in the journal Nature Medicine found that sleep deprivation causes the bursts of electrical activity that brain cells use to communicate to become slower and weaker. "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," reports NPR. From the report: 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. 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.