Sleep may strengthen memory

Sleep seems critical to memory, particularly the ability to recall recently learned facts and events, researchers report.

Known as "declarative" memories, these differ from non-declarative memories, or "how to" memories - those have already been shown to benefit from sleep. However, whether sleep has an impact on declarative memories has not been known.

This new finding may be particularly important for people with mentally demanding lifestyles, such as doctors, medical residents and college students, who often do not get enough sleep, the researchers say.

"We sought to explore whether sleep has any impact on memory consolidation," said lead researcher Dr Jeffrey M. Ellenbogen, a postdoctoral fellow at Harvard Medical School's Centre for Sleep and Cognition. "Specifically, the type of memory for facts and events in time."

The report is published in the July 11 issue of Current Biology.

How the study was conducted
Ellenbogen's team studied 60 people who did not use prescription drugs and did not have known sleep disorders or abnormal sleep patterns. Among these, 48 were assigned to one of four groups: sleep before testing, wake before testing, sleep before testing with interference, or wake before testing with interference.

As Ellenbogen explained, "interference is the concept in memory research that learning some new piece of information leads to the forgetting of something else, particularly when that something else is very similar."

In the study, everyone first attempted to memorise 20 paired words. They were tested 12 hours later for recall by completing a cued-recall task. However, people in the interference groups were also schooled in a second list of 20 word-pairs just before testing - these were the "distracting" or interfering words that made remembering the first bunch of word-pairs even tougher.

In addition, another 12 people were placed on a longer, 24-hour program with either interference and sleep or wakefulness.

Sleep linked to better recall
Ellenbogen's group found that sleep did have a benefit for declarative memory. People in the non-interference groups had mean recall that was slightly higher in the sleep group compared with the wake group. Moreover, people in the interference group who were able to sleep still did significantly better on the recall than did the wake group.

"Sleep had a benefit for the consolidation and strengthening of memory," Ellenbogen said. "It actively does so; it's not a passive process. The brain actively engages memories and leads them to be strengthened the next day, and it's a long-lasting benefit. The benefit was even larger than we were anticipating."

Given these findings, the researchers believe that sleep is important to building and maintaining memories. "Sleep is not an inactive state. That's an obsolete concept," Ellenbogen said. "The brain is doing lots of things during sleep, including consolidating memories. So you need to get sleep on a regular basis in order to maximise memory."

Sleep may assist learning
One expert thinks this study shows that sleep is important in learning.

"Sleep specialists still do not know the overarching purpose of sleep," said Dr Robert D. Vorona, an associate professor in the Division of Sleep Medicine at Eastern Virginia Medical School, in Norfolk. "However, we do know that insufficient sleep is associated with negative alterations in both mood and performance."

A number of studies suggest that sleep plays an important role in effective memory acquisition, Vorona said. "This study suggests that parents of students would do well to recommend that their children both study hard and obtain sufficient sleep in order to maximise their academic performance," he added. – (HealthDayNews)

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July 2006