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It's MORNING EDITION from NPR News. Good morning. I'm Steve Inskeep.

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And I'm Linda Wertheimer. In Your Health today: mouth guards for sleep apnea. But first, what is normal sleep, and how do we get it? Research suggests that shifting to two rounds of shut eye, with a period of wakefulness in the middle of the night, is a natural tendency for some people. NPR's Allison Aubrey reports.

ALLISON AUBREY: If you've ever been camping, sleeping under the stars, when suddenly you wake up and sort of lie there awhile quietly, then you doze off again 'til morning - well, that sort of divided sleep is similar to what many Colonial Americans likely experienced. Here's psychiatrist Tom Wehr.

Dr. TOM WEHR (Psychiatrist): There are historical records of people sleeping in two bouts at night. They called the first bout first sleep, or dead sleep, and the second bout, they called morning sleep. And the period in between, they referred to as watch, or watching.

AUBREY: Wehr points out that before artificial lighting, a winter day could bring 14 hours of solid darkness. People lived from sun to sun. He was curious. What might happen if he put busy Americans into that environment? How would they sleep? To find out, he and his colleagues at the National Institute of Mental Health recruited 15 healthy adult volunteers, who went about their normal business during the day, then reported to a sleep lab in the early evening.

Dr. WEHR: We had our subjects basically go into the dark at 6 o'clock at night, lie down and rest, and then get up at 8 o'clock in the morning. So it was a kind of simulated winter day.

AUBREY: What Wehr found is that the long nights had the same effect on his subjects as they had on colonial folks: two bouts of concentrated sleep, with a wakeful period in the middle - say, at 1 or 2 a.m. - lasting a few hours. And Wehr says interestingly, most of the participants described it as a time of quiet calm.

Dr. WEHR: You might think that lying awake for two hours in the middle of the night would be a kind of torture, but it wasn't at all. It was a - it's a kind of a quiescent, almost meditative state.

AUBREY: The notion of staying in bed, or in the dark, for 12 hours may seem ridiculous. We have busy lives, and experts say it's no surprise that in our 24/7 culture, we've adapted to a more efficient way of sleeping, consolidating it into one long stretch. This works for most people, but as we age, sleep changes.

Dr. MARY CARSKADON (Director of Chronobiology, Brown University): Sleep tends to be more fragile, in general, as we age.

AUBREY: Mary Carskadon is the director of chronobiology and sleep research at Brown University. She explains one way to assess the age-related changes in sleep is to look at brain waves.

Dr. CARSKADON: When we're little, we have a lot of very high, slow brain waves at the beginning of the night. And that seems to be the best, most restorative kind of sleep.

AUBREY: But as the decades go by, these peaks diminish. So if adolescent brain waves are the Himalayas, then by early adulthood, think Rockies, and in the elderly, Appalachians - or just foothills.

Dr. CARSKADON: And so it's easier to wake us up because those high, slow waves are very protective for disturbances in the environment.

AUBREY: Say a snoring partner or a barking dog. So the question is, what can we do about it? One option is to embrace a longer night. Sleep expert Jack Edinger of Duke University points out this really only works for people who are retired or have very flexible schedules.

Dr. JACK EDINGER (Sleep Expert, Duke University): They can afford, arguably, to spend more time in bed and experience this middle-of-the-night wake time because they can then stay in bed later in the morning and have that second bout of sleep.

AUBREY: There's also the siesta model, with naps in the afternoon. But for all the rest of us who need to get up and work all day, Edinger says there are strategies that can help people consolidate sleep.

One technique is to tightly restrict the number of hours in bed, sort of encouraging efficient sleep. Edinger stresses that sleep requirements vary, with six to nine hours being the normal range.

Dr. EDINGER: There are people that fall outside of that range and do fine.

AUBREY: Edinger says the important thing is to get a good handle on the dose of sleep you need, set your pattern, and stick with it each night. Most of us will find that's about seven or eight hours. Studies suggest that people who get less than seven hours tend to be more susceptible to the common cold and weight gain.

Allison Aubrey, NPR News, Washington.

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