You know you feel lousy when you haven’t gotten enough sleep, but you probably didn’t know why—and neither did scientists. Although anyone who’s pulled an all-nighter or stayed up with a screaming baby or otherwise failed to get a good seven hours of shut-eye can testify to how grumpy they are the next morning, exactly what, in the brain, was behind that grumpiness was anyone’s guess. Now it can be told: your prefrontal cortex has gotten disconnected from your amygdala.
So find scientists who kept 13 healthy volunteers up for 35 hours straight, for one day and the following night and another day, and compared them to 13 similar people who slept during that intervening night. At the end of day two, all 26 had their brains scanned with functional magnetic resonance imaging (fMRI), while they were shown 100 images that fell along an emotional gradient from neutral to very upsetting, from images of leaves and wicker baskets to pictures of mutilated bodies, severed limbs and children with grotesque tumors.
In the sleep-deprived, seeing gruesome images led to a noticeable spike in activity in the brain’s amygdale, the structure that tags incoming information with an emotion, especially a negative one—labelling a sight or sound frightening or disgusting. On no sleep, the scientists from the University of California, Berkeley, and Harvard Medical School will report online tomorrow in the journal Current Biology, the amygdala goes into overdrive, and the usual braking mechanism is broken.
“It’s almost as though, without sleep, the brain had reverted back to more primitive patterns of activity, in that it was unable to put emotional experiences into context and produce controlled, appropriate responses,” said Matthew Walker, director of Berkeley’s Sleep and Neuroimaging Laboratory and lead author of the study.
Tracing the patterns of firing showed why. Usually, signals from the amygdala reach the prefrontal cortex and vice versa. Since the prefrontal cortex is the site of logical reasoning, it can take the amygdala’s terror and calm it down by pointing out that, say, the frightening sound coming from the ceiling is just branches blowing in the wind, or that a terrifying scene in a movie is just celluloid.
In sleep-deprived brains, the fMRI showed, the amygdala connects more strongly to the locus coeruleus. This evolutionarily-ancient part of the brain, rather than acting as an emotional brake the way the prefrontal cortex does, steps on the accelerator: it releases noradrenalin, ramping up the brain’s jumpiness and emotionality even further. “The emotional centers of the brain were over 60 percent more reactive under conditions of sleep deprivation than in subjects who had obtained a normal night of sleep,” Walker said. “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.”
As a result, that friendly, reasonable co-worker turns into what he calls “emotional Jell-O” after an all-nighter, or else has to make a Herculean effort to rein in those emotions.
“You can see it in the reaction of a military combatant soldier dealing with a civilian, a tired mother to a meddlesome toddler, the medical resident to a pushy patient,” says Walker. Next step: figuring out how sleep deprivation cripples the emotional brain's connections to the rational brain.