Sleep Boosts Memory in Different Ways
Memories That Weaken by Day, Strengthen by Night
By Jennifer Warner
WebMD Health News
Reviewed by Brunilda Nazario, MD
Oct. 8, 2003 -- Memories that are lost during the day may actually come back after a good night's sleep. But new research suggests that not all memories are equal, and some may more likely be forgotten than others.
Two new studies in the Oct. 9 issue of Nature reveal that sleeps play a vital role in memory and learning that researchers are only now beginning to comprehend.
"Memory research is undergoing a transformation," writes researcher Karen Nader of McGill University in Montreal, in a commentary that accompanies the studies. "No longer is memory thought to be a hardwiring of information in the brain. Instead, it seems to be a process of storage and re-storage."
Sleep Restores and Improves Memory
In the first study, researchers found that sleep can rescue memories that were lost during the day.
The study showed that the ability of students to retain knowledge about words improved after one night's sleep, even if the student lost some of that knowledge during the day.
Researchers tested the students' ability to recognize a group of common words altered by a voice synthesizer that made them difficult to understand. Then they trained them to recognize the words and tested them at different intervals to see how effective the training was. Researchers say the task was like learning to understand someone speaking in a foreign language.
The study showed that performance declined over the span of a day but completely recovered after a night's sleep.
"We were shocked by what we found," says researcher Howard Nusbaum, professor of psychology at the University of Chicago, in a news release. "We were particularly intrigued by the loss of learning the students experienced during the day and then recovered."
The authors write that sleep has a function on learning. It consolidates memories, protecting memories from decay; sleep also appears to restore memory.
Storing Memories Is a Delicate Process
In the second study, researchers found new evidence that the process of storing memories may involve several stages, and memories may become vulnerable to loss during certain stages of the process.
The study looked at the effects of sleep on memory of a learned task -- different finger-tapping exercises at various intervals.
Researchers found that when a second finger-tapping exercise was learned immediately after the first, the memory of the second sequence interfered with the processing of the first learned exercise. In other words, the short-term memory of the first exercise was lost in the process of immediately learning a second exercise. Sleep enhanced the memory of the exercise, but only of the second learned task and not the first. This is because memory of the second learned exercise was not interfered with.
But when the second exercise was learned six hours after the first, it did not interfere with the processing of the first, and performance and memories of both were enhanced by sleep.
Researcher Matthew P. Walker and colleagues, of Harvard Medical School, say the findings show that when memories are recalled, they shift back to an unstable state. Once reactivated, these memories may either be restored or lost if this process is interfered with.
Nader says, "The finding that reconsolidation occurs in humans is a landmark discovery."
She says that the accuracy of the finger-tapping tests was affected by interference, but the speed with which they were performed was not. That suggests that perhaps not all memories undergo reconsolidation, or at least some memories might be affected by interference in different ways.
If further studies allow researchers to better understand the intricacies of this process, Nader says it could pave the way for new treatments for psychological disorders, such as posttraumatic stress disorder (PTSD) or addiction.
http://www.webmd.com/news/20031008/sleep-boosts-memory