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Many studies have shown that sleep plays a critical role in creating lasting memories. During shuteye, your brain replays the day's activities and stores away bits of data – not just facts and figures but also your emotional responses to those experiences.

There are, however, some things that are best forgotten. Unsettling memories of an emotionally disturbing event – such as being the victim of a crime or witnessing a gruesome accident – can trigger post traumatic stress disorder, or PTSD.

So, does the link between memory and sleep mean that PTSD can be prevented or reduced by simply not going to bed for an extended period after a traumatizing experience? Researchers at the University of Massachusetts, Amherst, think so.

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The team, led by Bengi Baran, recently completed a study that suggests emotional memories can be modified by altering the timing of sleep.

The study involved 68 female and 38 male volunteers between the ages of 18 and 30. They were shown a series of pictures, which included some disturbing images – such as a bloody crime scene and people with guns placed to their heads. The volunteers were asked to rate their emotional responses to the images.

Twelve hours later, the participants were shown another series of pictures that included some of the earlier disturbing photos. Once again they were asked to rate their reactions.

One group of volunteers saw both sets of images the same day. Those in the other group were exposed to the first set late at night and they saw the second set early the next morning.

The findings, published in The Journal of Neuroscience, revealed that those who went to sleep in between viewing sessions had a more lasting negative impression of the gory images. In fact, they were equally upset the second time they saw the pictures, said Rebecca Spencer, the senior author of the study.

On the other hand, those who saw all the images the same day – with no sleep in between – had "a greatly reduced emotional reaction" the second time they saw the negative pictures, she said.

For Dr. Spencer the message is clear: Sleep helps to preserve, and reinforce, those unpleasant memories.

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Why would a prolonged period of wakefulness after a shocking event lead to weaker emotional memories? With the passage of time, "you are piling more information into your brain and it might interfere with the initial reaction," she speculated.

More research will be needed to confirm the study findings. After all, the volunteers were looking at pictures, not witnessing a real traumatizing event. And the researchers can't yet say how long a person would need to stay awake to minimize the chances of developing PTSD.

But Dr. Spencer noted that after an unsettling experience, a lot of people have difficulty sleeping. It's "almost as if your brain doesn't want to sleep on it," she said. "The insomnia might be a healthy reaction."

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