You may think you're doing nothing at night, but to your brain, sleep means finally having some spare time to take stock of the day's events. Freed from the distractions of recording new experiences, a deeply sleeping brain can organize and strengthen memories, especially emotional ones.

For Katherina Hauner, a neuroscientist at Northwestern University Feinberg School of Medicine in Chicago who studies fear, that makes sleep a fascinating frontier. Hauner's latest research, just published in Nature Neuroscience, explores the connections between fear, memory, and sleep.

Walk me through your study in layman's terms.

The subjects were all healthy adults. While awake, they looked at pictures of faces with neutral expressions and learned to associate these with a mild electric shock, so that eventually these face pictures elicited a fear response in the brain. This is called fear conditioning.

Sounds fun.

Just to clarify, these were not painful shocks! They were simply startling, like you might get from opening a car door.

And we included an additional stimulus during this fear conditioning: smell. Each face was associated with a neutral smell, like mint or lemon, so both the faces and the smells became associated with the fear response.

Then, when subjects were asleep, we exposed them continuously to one of the smells again—one smell per person, chosen at random. The idea was to initiate the process of fear extinction.

What's "fear extinction"?

When a feared stimulus is presented again and again until the fear response decreases. It's the principle behind exposure therapy. Obviously it's much easier if that feared stimulus isn't actually threatening. My most recent research, before this, was about how the brain changes after a patient has been successfully treated for a spider phobia using exposure therapy.

How does exposure therapy work? People just look at a lot of spiders and get over their fear?

It's basically just slowly, slowly approaching the thing you're afraid of, with a therapist who's demonstrating each step before you do it. In the case of spiders, you learn to slowly move them around, and you learn that they're predictable, controllable, and not out to get you. Unfortunately, I think most people are unaware that their lifelong phobias can be treated so effectively. And it's quick! In that experiment, people with a lifelong terror of spiders were holding a tarantula in their hand within two hours. We saw immediate changes in the brain. And six months later, they still weren't afraid.

That's fascinating. Did you see evidence of fear extinction in this new experiment?

When the subjects awoke, we showed them all of the face pictures again, and measured their fear response from the amount of sweat on their skin. When they saw the target—the one face associated with the smell that they had experienced during sleep—their fear response was less than it had been before sleep. I should note that it was only a small decrease, though. If we'd asked people whether they felt less afraid, I don't know if they would have said they noticed a difference.

We also used MRIs to look at changes in the brain. After the sleep manipulation, we noticed that when subjects viewed the target, there were decreased responses in the hippocampus, an area of the brain that deals with processing memories. There were also changes in patterns of activity in the amygdala, which is associated with fear.

Does that mean you might be able to connect a smell to a picture of something a person fears deeply—say, a spider—and use this process to treat phobias during sleep?

Maybe, yes, although that's not yet been done. This is a very novel area of research, and a lot more work is needed before we could begin to talk about clinical applications. We haven't invented a new therapy. This was just one study done in one day, and these were fears and memories induced in the lab, not preexisting. But every new treatment has to start somewhere. My degree is actually in clinical psychology, and it's fascinating to me how quickly and effectively phobias can be treated, compared to everything else. If we can get a better idea of what's going on with phobias and why exposure therapy works so well for this disorder, maybe some of the mechanisms could be better applied to other disorders.

Like what?

Maybe PTSD [post-traumatic stress disorder]. It's an increasingly relevant problem for a lot of reasons—military engagement, gun violence, natural disasters—and I think entirely new treatments need to be developed.

Experiments like this remind me of the movie Inception. Should we fear scientists manipulating people's memories in their sleep?

I think the word "manipulate" has negative connotations involving deception, and that really couldn't be further from what any researcher is interested in doing. The term "manipulation of memory" can be thought of as a purposeful change in memory, which can be good or even necessary. So maybe "enhance" is a better word.

It can apply to physical learning, too, not just emotional memories. Part of my paradigm was inspired by other research going on at Northwestern that used similar procedures to enhance memories during sleep, to help people remember how to play a sequence of notes on a piano.

Does it matter what stage of sleep people are in?

Yes, most memory consolidation seems to occur specifically during slow-wave sleep, otherwise known as deep sleep.

So, reversing the idea, does this mean that lack of deep sleep makes it harder to remember things?

Yes, if people are woken up during slow-wave sleep, their memory for something recently learned turns out to be not as good. Older people tend to have less slow-wave sleep, so there's research being done to see if enhancing slow-wave sleep can improve memory.

Is there anything else you'd like to add?

If I had to give advice to readers---not based on this study so much as on sleep research in general--I would say this: We know that sleep is important for strengthening new memories, especially emotional ones. So if you're experiencing some negative or fearful event, if possible, don't go to sleep right away. Wait until you've worked through it and calmed a bit before going to sleep. This way you will have a less distressing version of that memory available for strengthening during sleep.

So don't fall asleep watching a scary movie.