Nov. 21, 2000 -- Sleep may, as Shakespeare wrote, "knit the raveled sleeve of care," but it also appears to weave the threads of experience into a solid fabric of memory and learning, say sleep researchers in the December issue of the journal Nature Neuroscience.
In two separate studies, researchers from the U.S. and Germany report that when we lose sleep, we also may lose the ability to store memories acquired during the day -- even after we've repaid a sleep "debt."
In other words, the next time you're caught napping on the job, you can tell the boss that you're just trying to commit an important task to memory.
"The average person requires about eight hours of sleep per night, but many otherwise healthy people continually deprive themselves of adequate sleep with consequences that include fatigue, poor decision making, and increased risk of accidents. New research ... demonstrating that sleep is required for memory consolidation may convince people to take sleep more seriously," writes John Spiro, PhD, assistant editor of Nature Neuroscience in an editorial accompanying the studies.
The foggy thinking that accompanies sleep deprivation may be caused by a failure of memory consolidation, the process by which training or experience is translated into improvements in performance, say Robert Stickgold, PhD, and colleagues at Harvard Medical School and the Massachusetts Mental Health Center in Boston.
The researchers put this theory to the test by training 18- to 25-year-old volunteers on a test that requires the user to identify specific letters and patterns as they are flashed briefly onto a video screen along with visual distractions.
After the initial training session, some of the volunteers were allowed to get a normal night's sleep, while others were kept awake until 9 p.m. the following evening, followed by two more nights of normal sleep. Some of the study participants were tested again within three hours of the first training session, while others were retested up to seven days later.
No one who was tested again on the same day had an improved score, but volunteers who enjoyed a good night's sleep after the initial test showed significant improvements in their scores, and performance continued to improve with subsequent nights of sleep. In contrast, volunteers who were forced to pull an all-nighter immediately following the test did not show any significant improvements, even after two nights of catch-up ZZZs.
The researchers write that "performance following a single training session improves beyond the first 24 hours, and improves more after a second night of sleep. We show that this improvement is absolutely dependent on the first night of sleep, and that subsequent sleep cannot replace the first night requirement."
The relationship between sleep and memories may be related to how adaptable the brain is to new learning, Pierre Maquet, PhD, senior research associate for the National Research Fund of Belgium, tells WebMD.
"You need a lot of sleep when you're an infant, because your brain is developing, and then you have a huge amount of [dream] sleep and [deep] sleep, and these are known to decrease with age, which suggests that something related to [brain] development is going on during sleep," says Maquet, author of an article on the nature of sleep that also appears in the journal.
If sleep is essential for memory consolidation, German investigators think they know when during the night memory processing and storage take place.
"We compared early and late sleep in humans, and ... what we found is that the early part of sleep is much more efficient," says Jan Born, MD, in the department of clinical neuroendocrinology at the Medical University of Lubeck.
Using a task similar to the one employed by the Harvard researchers, Born and colleagues compared the effects of early sleep, which is predominantly deep or "slow-wave" sleep, with late-phase sleep, which is primarily in the form of rapid-eye movement or "REM" sleep, when the most vivid and memorable dreams occur.
The researchers either trained volunteers in the evening and tested them after three hours of early deep sleep, or trained them in the middle of the night and retested them after three hours of late REM sleep.
They showed that retest performance improved significantly after deep sleep, but not after REM. As their American colleagues had, the German researchers also found that performance improved significantly after an uninterrupted night's sleep.
Born tells WebMD that the link between deep sleep and learning is suggestive rather than definitive. "Early sleep is the phase of sleep where slow-wave sleep is predominant," he says, "but there are a number of other changes going on in this sleep stage -- [brain hormone] changes that are also closely linked to slow-wave sleep, so from our experiments we can assume a link to slow-wave sleep, but we cannot prove such a link."