US researchers suggest smells could be used to calm fears - while people sleep.

美国研究人员建议，在人们睡觉的时候可以使用气味驱赶恐惧。

People were trained to associate two images, linked to smells, with fear.

During sleep they were exposed to one of those smells - and when they woke they were less frightened of the image linked to that smell.

A UK expert praised the Nature Neuroscience study and said it could help treat phobias（恐怖） and perhaps even post-traumatic stress disorders.

People with phobias are already commonly treated with "gradual exposure" therapy while they are awake, where they are exposed to the thing they are frightened of in incremental degrees.

This study suggests that the theory could be extended to therapy while they are in slow-wave, or deep, sleep.

This is the deepest period of sleep, where memories, particularly those linked to emotions, are thought to be processed.

Brain changes

The researchers showed 15 healthy people pictures of two different faces.

At the same time they were given a mild electric shock. They were also exposed to a specific smell, such as lemon, mint（薄荷）, new trainers, clove or wood.

They were then taken into a sleep lab. While they were in slow-wave sleep they were exposed to a smell linked to one of the faces they had been shown.

Later, when they were awake, they were shown both faces - without the scents or shocks.

They showed less fear when shown the face linked to the scent they had smelt while asleep than when shown the other face.

Their response was measured through the amount of sweat on the skin and fMRI (functional magnetic resonance imaging) brain scans.

These showed changes in the areas linked to memory, such as the hippocampus, and in patterns of brain activity in regions associated with emotion, such as the amygdala. 

People were in slow-wave sleep for between five and 40 minutes, and the effect was strongest for those who slept for longest.