Dreaming may act like a type of overnight therapy, taking the edge off painful memories, a new study says.

In a recent experiment, brain scans of people who viewed emotionally provocative pictures and then went to sleep showed that the part of the brain that handles emotions powered down during rapid eye movement, or REM, sleep—the stage in which dreams occur.

What's more, the subjects reported that the images had less of an emotional charge the morning after. This suggests that REM sleep may help us work through difficult events in our lives, the researchers say.

Why we sleep is still unknown, and even more elusive is the relationship between sleep and our emotional well-being, said study leader Matthew Walker, a neuroscientist at the University of California, Berkeley.

(Read about the mysteries of why we sleep in National Geographic magazine.)

There's already anecdotal evidence for sleep's therapeutic benefits—such as the oft-repeated adage that a person will go to bed and feel better in the morning, Walker said.

And clinical data show that psychiatric mood disorders, from anxiety to post-traumatic stress disorder, can lead to sleep abnormalities.

"Despite that suggested interplay, we've understood remarkably little about the basic brain science that may underlie a relationship between our emotional lives and our sleeping lives," he said.

As his new research now suggests, "it's not time that heals all wounds—it's REM sleep."

Sleeping on It Helps

For the experiment, Walker and colleagues divided 34 healthy young volunteers into two groups. People in each group viewed and rated their reactions to 150 images shown at 12-hour intervals while an MRI scanner measured brain activity.

The pictures, which have been used in hundreds of studies, ranged from bland objects—i.e., a kettle on a counter top—to gory pictures of people maimed in accidents, Walker said.

One group viewed the pictures in the morning and again in the evening without sleeping in between. The other group saw the same images before a full night of sleep and again the next morning.

The volunteers who slept between viewings reported a much milder emotional reaction to the images after the second viewing.

MRI scans performed during REM sleep revealed that brain activity fell in the amygdala—the emotion-processing part of the brain—possibly allowing the more rational prefrontal cortex to soften the images' impact. (See an interactive brain map.)

In addition, recordings of the subjects' electrical brain activity during sleep made with electroencephalograms showed a decrease in the levels of brain chemicals linked to stress.

When people experience an emotional event, stress chemicals are released to flag and prioritize that event, essentially reminding the brain to work through it during sleep, according to Walker, whose study appeared November 23 in the journal Current Biology.

"Somewhere between the initial event and the later point of recollecting, the brain has performed an elegant trick of divorcing emotions from memory, so it's no longer itself emotional," Walker said.

"That's what we mean by overnight therapy."

Dreaming Not an Emotional Cure-All?

But sleep expert David Kuhlmann said the team may have "overstepped its bounds slightly on the conclusions."

For instance, dreaming is not a cure-all for emotional stress, said Kuhlmann, medical director for sleep medicine at Bothwell Regional Health Center in Sedalia, Missouri.

Though dreaming "helps improve the ability to handle stressful situations," it's unlikely that REM sleep will make bad memories totally dissipate, he said.

However, he said "it certainly is an interesting finding in this limited subject size in an artificial environment."

Both experts agreed that sleep is often ignored in the medical profession, even as research has been revealing its multiple health benefits.

And study leader Walker noted that the latest work is "yet another reminder that sleep is not a state where our brain is dormant and not doing anything."