Meeri Kim / Special to The Washington Post

It can take only an instant for fear to take hold in the brain. A fear of snakes after being bitten by one, or feeling anxious around bodies of water after witnessing a drowning.

Overcoming that fear can take a long time, but now researchers are saying it can be done in your sleep. Scientists at Northwestern University say they have successfully lowered levels of fear in humans by using certain odors to trigger and reassociate frightening memories into harmless ones during a deep slumber.

“Sleep sort of stamps memories in more strongly,” said neurologist Jay Gottfried, senior author of the study, which was scheduled to be published online Sunday by the journal Nature Neuroscience. “That’s when a lot of memory formation can take place.”

The researchers first created a fear of a certain face in their subjects by using conditioning — making them link together a certain face and smell in their minds with a painful electric shock. After some trials, the participants became afraid of the face, and the smell acted as a cue associated with that face.

The researchers then used the smell to trigger fear memories during sleep as a way to acclimate patients without the stress of conscious terror.

Gottfried and his colleagues have not attempted this on pre-existing fears, but theoretically it could be done — by creating a connection between a phobia and a distinct smell. He said the first kind of patients who could be helped by this process would be those who already have a smell associated with their fear — for example, the smell of gunpowder.

“From a clinical perspective, this can be a new approach to try and treat stressful or traumatic memories,” Gottfried said.