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Sleep Clears Way for New Learning

New Research May Help Explain How Brain Changes That Occur During Sleep Affect Learning

By Salynn Boyles
WebMD Health News

Reviewed By Louise Chang, MD

April 2, 2009 -- Want to learn something new? Try getting a good night's sleep or taking a long nap.

Sleep is now recognized as being critical for learning and memory, and now a new study in fruit flies offers clues as to why.

The research found that, in fruit flies at least, sleep weakens and even destroys existing synapses within the brain to make room for the creation of new ones.

Neurologists believe that the brain encodes memories and learning by creating these new synapses, which connect information-processing neurons throughout the central nervous system.

But seemingly conflicting theories exist about how sleep affects important synaptic connections; researcher Paul J. Shaw, PhD, of St. Louis' Washington University tells WebMD.

"It is clear that if you don't sleep, you don't acquire information very well, you don't process it very well and you don't store it very well," he says. "But we haven't really known why that is. One theory is that sleep strengthens synapses and another is that it weakens them."

People, Flies Need Sleep to Learn

Shaw says studying fruit flies in an effort to understand the human sleep-learning connection is less of a leap than one might think.

"We know that babies sleep a lot, and so do very young flies and that, like humans, if you deprive flies of sleep they do not learn," he says. "If you had asked me 10 years ago if we would have found as many similarities between the fly and human brain, I would have said, 'No way.'"

In earlier research, he and colleagues genetically altered fruit flies to make it possible to track the development of new synapses. They discovered that a very small number of neurons -- 16 out of about 200,000 -- are involved in the formation of new memories.

They also discovered that flies needed more sleep after social interaction experiments. When the flies slept, the number of new synapses formed during the social experiments declined and when the flies were deprived of sleep, the declines did not occur.

In their latest research, published in the April 3 issue of the journal Science, the researchers identify three specific genes critical to the sleep-learning connection.

One of these genes is the fly equivalent of a specific gene in humans linked to learning and memory, known as the serum response factor.

To Sleep, Perchance to Learn

The findings may help explain how sleep affects brain circuitry to clear the way for new learning, but Shaw does not believe they explain everything.

He says while some key synapses may be weakened or destroyed during sleep, others may be strengthened.

Sleep researcher Sara C. Mednick, PhD, tells WebMD that the fruit fly research offers important clues about how sleep may affect learning in humans.

Mednick's own research focuses on the impact of napping during the day on health, memory, and learning.

She says different stages of sleep seem to be needed for different types of learning.

Mednick explains that a typical sleep cycle involves 10 to 20 minutes of drowsiness and very light sleep, followed by an hour or so of very deep sleep, followed by REM period, when dreaming occurs. Over the course of a night, the cycle repeats, with the length of deep sleep decreasing and REM sleep increasing.

While 20-minute "cat naps" or even quiet meditation may help the brain process and learn simple information like the historic dates they might encounter on a test, processing more complex information and learning new skills seems to require deeper sleep, Mednick says.

Mednick is an assistant professor of psychiatry at the University of California, San Diego.

"A lot of people are walking around sleep deprived, and the 20-minute nap can really help to shake off that middle-of-the afternoon exhaustion," she says. "But it appears that longer sleep is needed for establishing the connections involved in creative or complex learning."

SOURCES: Donlea, J.M., Science, April 3, 2009. Paul J. Shaw, PhD, assistant professor of neurobiology, Washington University School of Medicine, St. Louis. Sara C. Mednick, PhD, assistant professor of psychiatry, University of California, San Diego.

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