By Joene Hendry

NEW YORK (Reuters Health) - Sleep tends to help people better remember aspects of a negative event while allowing memory of background information to fade, researchers have found.

The brain "seems to make adaptive 'decisions' about what to remember and what to forget," Dr. Jessica D. Payne of Beth Israel Deaconess Medical Center and Harvard Medical School in Boston, Massachusetts, told Reuters Health. This suggests the sleeping brain does more than simply consolidate whatever one puts into it, Payne said.

Payne and colleagues enlisted 88 college students to participate in recall tests after seeing pictures that depicted either neutral subjects on a neutral background (a normal 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.

Some of the students viewed the pictures in the morning and took memory recall tests 12 hours later after a full day and no napping. Others viewed the pictures at night, slept for 12 hours, and completed memory recall tests in the morning, while a third group of students viewed the pictures either in the morning or the evening and completed recall tests 30 minutes later.

The results suggest that sleep tends to promote better memory of objects that evoke emotions.

Overall, memory for negative objects was enhanced 68 percent by a sleep period compared with 44 percent by a wake period, while memory for accompanying backgrounds was the same (38 percent) after the sleep or no-sleep tests, the study team reports in the medical journal Psychological Science.

"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 noted in a written statement. But, among the subjects 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," said Payne. "You might say that sleep is actually working at night to decide what memories to hold on to and what to let go of."

SOURCE: Psychological Science, August 2008

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