Nightmare: we may be reliving scary daytime events Chris Dale/Getty

Have we had our first peek at the source of nightmares? When rats are given a fright while they are awake, the fear centre of their brains gets reactivated when they next go to sleep. This could explain why people who go through frightening experiences often have nightmares afterwards, says György Buzsáki of New York University.

Rats store mental maps of the world they experience in their hippocampi – two curved structures in the brain. Different places are processed by distinct groups of neurons in the hippocampi that fire together in sequence as rats run around a maze, for example.

Later, after exploring an environment like this, these firing sequences have been seen replaying as the animals sleep, as if dreaming of the routes they’d taken. This process is thought to allow memories to become consolidated for longer term storage, and has recently been detected in people for the first time.


Buzsáki’s team wondered if such memory replay might include not just spatial information but also how the animal was feeling at the time. They tested this by giving a rat an unpleasant but harmless experience – a puff of air in the face from a computer keyboard cleaner – at a particular spot along a route.

Fear revisited

As expected, the rats learned to fear that particular place. “They slow down before the location of the air puff, then run superfast away from it,” says Buzsáki’s colleague, Gabrielle Girardeau. “If you do it in the face of a human, they don’t like it either.”

As well as looking at the animals’ hippocampi, the team also recorded activity in their amygdalas, a pair of nearby structures in the brain that become active when we are scared.

Sure enough, when the rats replayed their memories of the route while sleeping, their amygdalas became more active at the same moment they mentally revisited the frightening spot. “This is the first [replay] study to bring in the emotional system,” says Dan Bendor of University College London. “That’s really important because our memories are not just information – we remember all the emotional context.”

It’s unclear if the rats experienced this replay of emotions as a dream, says Buzsáki. “We can’t ask them.” But he does think that if the same thing goes on in people, it might lead to nightmares. “It has been fairly well documented that trauma leads to bad dreams,” he says. “People are scared to go to sleep.”

But James Bisby of University College London thinks that’s too big a leap. “It seems to be more of a memory strengthening process. The memory would be more likely to be retrieved because it’s stabilised.”

Journal reference: Nature Neuroscience, DOI: 10.1038/nn.4637