Sleep Disorders

25 November 2008

Naps good for memory

Just in time for the holidays, some medical advice most people will like: take a nap - even a 12-minute nap can boost some forms of memory.

Just in time for the holidays, some medical advice most people will like: Take a nap. Interrupting sleep seriously disrupts memory-making, compelling new research suggests.

But on the flip side, taking a nap may boost a sophisticated kind of memory that helps a person see the big picture and get creative.

"Not only do we need to remember to sleep, but most certainly we sleep to remember," is how Dr William Fishbein, a cognitive neuroscientist at the City University of New York, put it at a meeting of the Society for Neuroscience last week.

Good sleep is a casualty of the busy modern world. Surveys suggest few adults attain the recommended seven to eight hours a night. Way too little clearly is dangerous: Sleep deprivation causes not just car crashes but all sorts of other accidents.

Chronic lack of sleep leads to serious health problems
Over time, a chronic lack of sleep can erode the body in ways that leave us more vulnerable to heart disease, diabetes and other illnesses. Perhaps more common than insomnia, however, is fragmented sleep, the easy awakening that comes with aging, or, worse, the sleep apnoea that afflicts millions, who repeatedly quit breathing for 30 seconds or so throughout the night.

Indeed, scientists increasingly are focusing less on sleep duration and more on the quality of sleep, what is called sleep intensity, in studying how sleep helps the brain process memories so they stick. Particularly important is "slow-wave sleep," a period of very deep sleep that comes earlier than better-known REM sleep, or dreaming time.

Fishbein suspected a more active role for the slow-wave sleep that can emerge even in a short power nap. Maybe the brain keeps working during that time to solve problems and come up with new ideas.

How the study was done
So he and graduate student Hiuyan Lau devised a simple test: documenting relational memory, where the brain puts together separately learned facts in new ways. First, they taught 20 English-speaking college students lists of Chinese words spelled with two characters, such as sister, mother, maid.

Then half the students took a nap, being monitored to be sure they did not move from slow-wave sleep into the REM stage. Upon awakening, they took a multiple-choice test of Chinese words they never had seen before.

The nappers did much better at automatically learning that the first of the two-pair characters in the words they had memorized earlier always meant the same thing - female, for example. So they also were more likely than non-nappers to choose that a new word containing that character meant "princess" and not "ape."

"The nap group has essentially teased out what's going on," Fishbein concludes.

These students took a 90-minute nap, quite a luxury for most adults. But even a 12-minute nap can boost some forms of memory, adds Dr Robert Stickgold of Harvard Medical School.

Sleep apnoea suppresses birth of new brain cells
Conversely, Wisconsin researchers briefly interrupted night-time slow-wave sleep by playing a beep - just loudly enough to disturb sleep but not awaken - and found those people could not remember a task they had learned the day before as well as people whose slow-wave sleep was not disrupted.

That brings us back to fragmented sleep, whether from ageing or apnoea. It can suppress the birth of new brain cells in the hippocampus, where memory-making begins, enough to hinder learning weeks after sleep returns to normal, warns Dr Dennis McGinty of the University of California, Los Angeles.

To prove a lasting effect, McGinty mimicked human sleep apnoea in rats. He hooked them to brain monitors and made them sleep on a treadmill. Whenever the monitors detected 30 seconds of sleep, the treadmill briefly switched on.

After 12 days of this sleep disturbance, McGinty let the rats sleep peacefully for as long as they wanted for the next two weeks.

The catch-up sleep did not help: Rested rats used room cues to quickly learn the escape hole in a maze. Those with fragmented sleep two weeks earlier could not, only randomly stumbling upon the escape.

'Get sleep problems sorted'
None of the new work is enough, yet, to pinpoint the minimum sleep needed for optimal memory. What's needed may vary considerably from person to person.

"A short sleeper may have a very efficient deep sleep even if they sleep only four hours," notes Dr Chiara Cirellia of the University of Wisconsin, Madison. But altogether, the findings do suggest some practical advice: Get apnoea treated.

Avoid what Harvard's Stickgold calls "sleep bulimia," super-late nights followed by sleep-in weekends. And don't feel guilty for napping. – (Sapa, November 2008) Read more: Smokers don't sleep well: study , Too much/little sleep makes you fat


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Dr Alison Bentley is a general practitioner who has consulted in sleep medicine and sleep disorders, in both adults and children of all ages, for almost 30 years. She also researches and publishes on a number of sleep-related topics both in formal research journals and lay publications including as editor of Sleep Matters, an educational newsletter on sleep disorders for doctors.

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