Nightmares help prepare our brains to handle stressful situations, according to a small US government-funded study.
Researchers from Switzerland and the US subjected 18 people to a bizarre experiment: they were fitted with electrodes to monitor their brain activity at night, and they were woken multiple times to answer interview questions, such as: 'Did you dream? And, if so, did you feel scared?'
The researchers found a pattern: during nightmares, there was often heightened activity in regions of the brain that control emotions.
In a second experiment, they gave 89 people a dream diary to fill out for a week. At the end, each person sat through an MRI scan while being shown negative and scary images.
They found that, in people who had experienced nightmares, their emotional brain regions responded faster and more efficiently than in those who had not.
Lampros Perogamvros, one of the lead authors, who works at the University of Geneva, said: 'We were particularly interested in fear: what areas of our brain are activated when we're having bad dreams?'
The first experiment in the study, partly funded by the National Institutes of Health and published last month in the journal Human Brain Mapping, helped them to identify how nightmares light up the brain in real time.
'By analyzing the brain activity based on participants' responses, we identified two brain regions implicated in the induction of fear experienced during the dream: the insula and the cingulate cortex', Perogamvros said.
'For the first time, we've identified the neural correlates of fear when we dream and have observed that similar regions are activated when experiencing fear in both sleep and wakeful states.'
The insula unleashes our fear response in times of danger, while the cingulate cortex controls it.
The second experiment helped to understand the psychological or physiological impact nightmares have on our brains.
Their conclusion was that, to an extent, nightmares seem to be beneficial, helped steel us for stressful experiences.
'We showed each participant emotionally-negative images, such as assaults or distressful situations, as well as neutral images, to see which areas of the brain were more active for fear, and whether the activated area changed depending on the emotions experienced in the dreams over the previous week,' Virginie Sterpenich, a neuroscience researcher at the University of Geneva, said.
Beyond the insula and cingulate cortex, they also looked at the amygdala and prefrontal cortex which also control emotion.
'We found that the longer a someone had felt fear in their dreams, the less the insula, cingulate and amygdala were activated when the same person looked at the negative pictures,' Sterpenich said.
'In addition, the activity in the medial prefrontal cortex, which is known to inhibit the amygdala in the event of fear, increased in proportion to the number of frightening dreams!'
However, traumatic and harrowing nightmares that cause sleeplessness and stress are not beneficial, and may be counterproductive, the researchers said.
'We believe that if a certain threshold of fear is exceeded in a dream, it loses its beneficial role as an emotional regulator,' Perogamvros said.