A new technique makes it possible to sleep away your fears.
The research, which was published in the journal Nature Neuroscience, showed for the first time that the power of emotional memories — specifically, fearful ones — can be weakened with sleep-based tactics, which offers hope that something as simple as a good night's slumber may reduce phobias and symptoms of posttraumatic stress disorder (PTSD).
Researchers led by Katherina Hauner, who conducted the study as a postdoctoral student at the Feinberg School of Medicine at Northwestern University in Chicago, studied 15 participants who were taught to fear images of particular men's faces by receiving mild but uncomfortable shocks whenever these faces were shown. Each of the target visages was accompanied by a recurrent odor that the participants chose because they didn't have any prior emotional associations. (Smells are especially linked with memories of feelings.)
The participants then took a nap. While they slept, the researchers repeatedly reintroduced the odors, including the ones linked to the target faces and a shock — but this time, without the shock.
It was obvious that the smells affected the sleepers. Although they weren't awake, when the fear-linked odors were wafted in their direction, their skin conductance, which measures emotional arousal, rose. But over time, with repeated exposure, that response declined. (The scientists intentionally chose smells that don't normally activate pain nerves — like strong peppermint — that can wake people up.)
When the participants faced the scary images after their nap, they were less likely to flinch. In other words, their fear response had been reduced while they slept. And the longer they slept and were exposed to the scent, the less afraid they were when they awoke.
"Individual memories related to fearful events can be specifically targeted and changed during sleep," says Hauner. "To my knowledge, this is the first [experiment] to show that emotional memories can be manipulated during sleep in humans."
While she cautions that this technique is not yet ready for clinical use, if other scientists repeat and investigate the process more deeply, it could one day be added to exposure therapy, which is the most effective treatment for phobias and is also used for PTSD. Exposure involves having people engage in their feared experiences gradually — while they are awake — until they learn not to overreact. But because they are conscious of having to face their fears, many patients refuse even to try it. If some of this exposure work could be done while they were asleep, more people might benefit from the therapy.
"[Exposure therapy] is extremely stressful, especially at the beginning," Hauner says. "It's very effective for specific phobias and not as good for PTSD. It can be a very difficult process, so anything we can do to enhance it would be good."
Why would sleeping on a fearful experience diminish its power? Researchers believe that one of the main functions of sleep is to consolidate memories so they can be stored to make room for new memories, therefore freeing up more brain capacity. (Dreams, in fact, may be the replaying of those memories during this processing and storage.)
One element of a memory involves emotion, which flags the memories that should be kept and those that are filed away or even deleted. But each time a memory is brought up, recalled and stored again, it can be changed in subtle ways. That's why an unconscious experience like smelling a scent during sleep can reshape the memory so its emotional poignancy — in this case, fear — is attenuated so the next time it is activated, it's less emotionally powerful.
Other research also suggests that emotional memories may be processed in different ways than neutral ones, and that sleep tends to reduce emotional intensity.
"In general, the idea is that maybe sleep helps to increase our memory but reduce our worries," says Bjorn Rasch, a professor of cognitive biopsychology at the University of Fribourg in Switzerland, who was not associated with the research, noting that this may be why anxiety disorders and depression are often accompanied by sleep problems.
The new study suggests we may be able to hack this mechanism to fight troubling memories — and get closer to one day sleeping them off.