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By Sarah Klein

SUNDAY, Feb. 21, 2010 (Health.com) — If you feel a little sluggish after lunch, a quick snooze can perk you up. According to a new study, it may even make you smarter.

The longer you're awake, the more difficult it is for your brain to store new information, whether it’s faces and names, the details of a conversation, or mental notes for a big presentation. An afternoon nap seems to refresh this short-term memory and free up space for new information, researchers found.

In the study, the researchers asked 39 college students to learn a series of new names and faces at noon and match the faces and names a few minutes later. They then performed the same test at 6 p.m. the same day. A group of students who took a 90-minute afternoon nap at 2 p.m. performed better than non-napping students, who had a serious decline in their memory test scores.

Why? The part of your brain where short-term information and memories are stored is a bit like your email inbox, says the study’s lead author, Matthew P. Walker, the head of the Sleep and Neuroimaging Laboratory at the University of California, Berkeley. "You can only receive so many emails before your inbox starts to bounce,” he says. “When you sleep, essentially what you may be doing is clearing out that inbox to another folder, [so] you have a refreshed capacity to receive new emails."

Walker and others have previously studied the harmful effects of sleep deprivation (such as all-nighters) on sleep and learning capacity. This study is among the first to demonstrate that the brain's ability to absorb new information declines over the course of a normal day, and that naps can reverse this decline, according to Walker, who presented his findings on Sunday at the American Association of the Advancement of Science’s annual meeting in San Diego. The research was funded by the National Institutes of Health.

Don't email this article to your boss to justify sleeping on the job quite yet. Neil Kline, DO, a board-certified sleep physician, says that while "the average reader will take away from this that taking a nap is a good thing and will improve memory," Walker's study has some caveats.

Next page: Naps may not be as effective for everyone

Dr. Kline points out that because college students as a group tend to get inadequate sleep, the memory capacity of the students in the study may have been below average. The nap likely was more beneficial for them than it would be for someone with a healthier sleep schedule.

"Typically students are sleep deprived because they’re up late studying and having a good time," Dr. Kline says. "As a result, they have the ability to take a nap and catch up on some of that sleep deprivation during the middle of the day." (In other words, the results shown in the study may not be typical.)

If you're getting adequate sleep at night, you shouldn't feel drowsy during the day, Dr. Kline adds. “Ideally, adults should not feel the need to take a nap," he says. "You should try to have a sleep schedule that keeps you alert during the daytime."

Walker agrees that people sleeping eight hours a night probably won't feel the need for a nap. But, he says, if you do feel sleepy during the day and are able to catch some shut-eye, do it. (Just make sure you don’t push the nap back too late in the day, he warns, since napping too close to bedtime can aggravate insomnia or other sleep disorders.)

The study also provides new information on how, exactly, the brain clears storage space to make room for new memories. Walker and his colleagues monitored the brain waves of the students in the nap group while they slept, and they discovered that better performance on the 6 p.m. test correlated with a certain type of sleep: stage 2 non-REM sleep, the stage between restful deep sleep and rapid eye movement (REM) sleep, or dream sleep.

“It tells you that something very specific is going on,” Walker says. “It wasn’t just the total amount of sleep, but a very particular type of sleep that was facilitating improvements.”

The study provides more evidence that humans may not be biologically engineered to sleep one long stretch at night and stay awake all day, Walker says. Instead, our natural rhythm may be to sleep fewer hours at night and take a long afternoon nap each day, as many people continue to do in "siesta cultures" and in societies with little electricity, Walker points out.

“We’ve all had that experience of being in meetings after lunch with people who are clearly drifting off,” he says. “It’s not their fault. It’s their biology!"