Besides helping you feel well-rested, getting your zzz's may also sharpen your memory, a new study shows.
Researchers found that sleep not only protects memories from outside interferences, it also helps strengthen them.
"There was a very large benefit of sleep for memory consolidation, even larger than we were anticipating," said study author Dr. Jeffrey Ellenbogen, an associate neurologist at Brigham and Women's Hospital, Boston, and a postdoctoral fellow in sleep medicine at Harvard Medical School.
The research is scheduled to be presented May 2 at the American Academy of Neurology annual meeting in Boston.
In the study, the researchers focused on sleep's impact on "declarative" memories, which are related to specific facts, episodes and events.
"We sought to explore whether sleep has any impact on memory consolidation, specifically the type of memory for facts and events and time," Ellenbogen said. "We know that sleep helps boost memory for procedural tests, such as learning a new piano sequence, but we're not sure, even though it's been debated for 100 years, whether sleep impacts declarative memory."
The study involved 48 people between the ages of 18 and 30. These participants had normal, healthy sleep routines and were not taking any medications. They were all taught 20 pairs of words and asked to recall them 12 hours later. However, the participants were divided evenly into four groups with different circumstances for testing: sleep before testing, wake before testing, sleep before testing with interference, or wake before testing with interference.
Two of the groups (the wake groups) were taught the words at 9 a.m. and then tested on the pairings at 9 p.m., after being awake all day. The other two groups (the sleep groups) learned the words at 9 p.m., went to sleep, and were then tested at 9 a.m.
Also, prior to testing, one of the sleep groups and one of the wake groups were given a second list of 20 word pairs to remember. These groups were then tested on both lists to help determine memory recall with interference (competing information).
The result: Sleep appeared to help particpants recall their learned declarative memories, even when they were given competing information.
According to the researchers, people who slept after learning the information performed best, successfully recalling more words whether or not there was interference. Those in the sleep group without interference were able to recall 12 percent more word pairings from the first list than the wake group without interference (94 percent recall for the sleep group vs. 82 percent for the wake group).
When presented with interference, those who slept before testing did significantly better at remembering the words (76 percent for the sleep group vs. 32 percent for the wake group).
"We were surprised to find the order of magnitude by which the data demonstrated our effects," Ellenbogen said.
Jan Born, a professor of neuroendocrinology at the University of Lübeck in Germany, said the study offers more proof of the importance of sleep for memory consolidation.
"Considering that learning in every educational setting (schools, colleges, etc.), is centrally based on hippocampus-dependent memory function [declarative memories], people should realize that optimal learning conditions require proper sleep," he said.
Proper sleep may have other benefits, too, added Michael Perlis, director of the Sleep Research Laboratory at the University of Rochester in Rochester, NY. Research has shown that in addition to memory, sleep may be related to physical functioning, good immune function, physical and cognitive performance, and mood regulation, he said.
"These are all theories. The only thing we know is that when we're deprived of sleep, we do less well. Is that a lack of sleep or sustained wakefulness? It's very difficult to figure out how to crack that nut," he said. "We spend 30 percent of our time on sleep. What is sleep for? This is a riddle we're still working on."
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