Led by researchers at Beth Israel Deaconess Medical Center (BIDMC) and Boston College, the study showed that a period of slumber helps the brain to selectively preserve and enhance those aspects of a memory that are of greatest emotional resonance, while at the same time diminishing the memory's neutral background details.
"This tells us that sleep's role in emotional memory preservation is more than just mechanistic. In order to preserve what it deems most important, the brain makes a trade-off, strengthening the memory's emotional core and obscuring its neutral background," said the study's first author Jessica Payne, PhD, a Harvard University research fellow in the Division of Psychiatry at BIDMC.
The researchers said that sleep's importance in the development of episodic memories - in particular, those with emotional resonance- has been less clear.
For the study, the researchers tested 88 college students. Study participants were shown scenes that depicted either neutral subjects on a neutral background (a car parked on a street in front of shops) or negatively arousing subjects on a neutral background (a badly crashed car parked on a similar street).
The participants were then tested separately on their memories of both the central objects in the pictures and the backgrounds in the scenes.
In this way, memory could be compared for the emotional aspects of a scene (the crashed car) versus the non-emotional aspects of the scene (the street on which the car had crashed.)
Subjects were divided into three groups. The first group underwent memory testing after 12 hours spent awake during the daytime; the second group was tested after 12 night time hours, including their normal period of night time sleep; and the third baseline group was tested 30 minutes after viewing the images, in either the morning or evening.
"Our results revealed that the study subjects who stayed awake all day largely forgot the entire negative scene [they had seen], with their memories of both the central objects and the backgrounds decaying at similar rates," Payne said.
However, she added, among the individuals who were tested after a period of sleep, memory recall for the central negative objects (i.e. the smashed car) was preserved in detail.
"Sleep is a smart, sophisticated process. You might say that sleep is actually working at night to decide what memories to hold on to and what to let go of," Payne said.
The study is published in the August 2008 issue of the journal Psychological Science.