The new study from Harvard Medical School and the University of California, Berkeley is the first to explain, at the neural level, what seems to be a universal phenomenon: that sleep loss leads to emotionally irrational behavior, according to the researchers. The findings might also offer some insight into the clinical connection between sleep disruptions and psychiatric disorders.
"This adds to the critical list of sleep's benefits," said Matthew Walker, from the University of California, Berkeley. "Sleep appears to restore our emotional brain circuits, and in doing so prepares us for the next day's challenges and social interactions. Most importantly, this study demonstrates the dangers of not sleeping enough. Sleep deprivation fractures the brain mechanisms that regulate key aspects of our mental health. The bottom line is that sleep is not a luxury that we can optionally choose to take whenever we like. It is a biological necessity, and without it, there is only so far the band will stretch before it snaps, with both cognitive and emotional consequences."
Scientists have known that sleep deprivation impairs a range of bodily functions, including the immune system and metabolism, as well as brain processes, such as learning and memory, the researchers explained. Yet, evidence for the role of sleep in governing our emotional brain state had remained surprisingly scarce, they noted.
In the new study, Walker's team assigned 26 healthy people to either a sleep-deprivation group—in which participants were kept awake for about 35 hours—or a normal sleep group. On the following day, the study subjects' brains were scanned by functional magnetic resonance imaging (fMRI), which measures brain activity on the basis of blood flow, while viewing 100 images. The images were at first emotionally neutral, but became increasingly aversive over time.
"We had predicted a potential increase in the emotional reaction from the brain [in people deprived of sleep], but the size of the increase truly surprised us," Walker said of the study's findings. "The emotional centers of the brain were over 60% more reactive under conditions of sleep deprivation than in subjects who had obtained a normal night of sleep. It is almost as though, without sleep, the brain reverts back to a more primitive pattern of activity, becoming unable to put emotional experiences into context and produce controlled, appropriate responses.
"While it is early days," he added, "clinical evidence has shown that some form of sleep disruption is present in almost all psychiatric disorders. These findings may offer new mechanisms as to why, and provide novel insights into how we can understand and even treat these disorders at a brain level."