© Joel Sartore/National GeographicNaps wipe the brain's memory slate clean, a new sleep study says.
That's the conclusion of a new study that may explain why people spend so many of their sleeping hours in a pre-dreaming state known as stage 2 non-rapid eye movement, or REM, sleep.
For years sleep studies have hinted that shut-eye improves our ability to store and consolidate memories, reinforcing the notion that a good night's sleep - and power naps - is much more conducive to learning than an overnight cram session.
Now scientists may have figured out how, in part, this happens: During sleep, information locked in the short-term storage of the hippocampus - the part of the brain responsible for memories - migrates into the longer-term database of the cortex.
This action not only helps the brain process new information, it also clears out space for the brain to take in new experiences.
That means "it's not just important to sleep after learning, it's critical to sleep before learning," study leader Matthew Walker, of the University of California, Berkeley, said today during a press briefing.
"Sleep prepares the brain like a dry sponge, ready to soak up new information."
Power Naps
In his latest work, presented today at a meeting of the American Association for the Advancement of Science in San Diego, Walker and colleagues asked 39 young adults to perform several tasks related to fact-based learning.
One group was then asked to take a 90-minute nap while the other group stayed awake.
Afterward, both groups engaged in a new round of tasks. The non-nappers performed much worse than the nappers, the researchers found.
Measuring the nappers' electrical activity in the brain revealed that their "cache" cleared during stage 2 non-REM sleep.
The new work not only backs up the benefits of midday naps, it also may have also solved the long-standing mystery of why people need stage 2 non-REM sleep.
Although the "dreaming" sleep stage known as REM is perhaps better known, humans actually spend about half of the night in stage 2 non-REM.
REM sleep is crucial for more complex thinking, such as making non-obvious connections between previously learned facts - a process he describes as "a Google search gone wrong - or right."
"When you have a problem, no one says you should 'stay awake on it,'" he quipped.
Instead, sleep - specifically the REM stage - is a way for the brain to take information that might at first seem unrelated to your mental "search" and come up with creative solutions.
In fact, he said, our dreams could be a testing ground for this subconscious problem-solving.
Not Everyone Built for Naps
Unfortunately, the new findings don't mean that all people would benefit from an afternoon siesta, noted Sara Mednick, a psychiatry professor at the University of California, San Diego.
Some nappers tend to wake up groggy and disoriented because of something called sleep inertia.
"This is when you wake up during deep, slow-wave sleep," she said today during the briefing. Since brain temperature and blood flow to the brain decrease during this sleep stage, it's jarring to suddenly be awake and experiencing much higher rates of brain activity.
Previous studies have shown that habitual nappers tend to be light sleepers. This means they spend much less time, at least during the first few hours of sleep, in a deep non-REM sleep state.
If naps leave you groggy, it's also possible to get a similar performance boost on some tasks simply by taking a mental time-out, she said.
"In some cases," she said, "quiet rest and naps give the same [memory] benefits."
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