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updated 2/1/2010 9:30:20 AM ET 2010-02-01T14:30:20

How much sleep we need is largely a mystery, and sleep seems tougher to come by as we age. Many studies — often funded by the pharmaceutical industry — have suggested that we're all sleep-deprived zombies, risking our health for lack of shut-eye.

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But new research in the U.K. confirms previous indications that older people need less sleep. It also suggests that variations in sleep hours needed are normal and healthy — so long as one is not overly sleepy during the day.

"Healthy aging appears to be associated with reductions in the sleep duration and depth required to maintain daytime alertness," the scientists said in a statement.

Still, researchers warn that many people in modern society suffer from sleep deprivation, and that it can lead to plenty of woes from accidents on the job to higher risk of falls and even death in elderly people.

The study, announced today, involved 110 healthy adults who did not have any sleep disorders and didn't complain about lack of sleep. They went through various rounds of sleep and wake periods under varying conditions, and were tested for sleepiness during the wake periods.

During the first night with eight hours in bed, the resulting average sleep time, by age group:

Age 20-30: 433.5 minutes (7.23 hours)
Age 40-55: 409.9 minutes (6.83 hours)
Age 66-83: 390.4 minutes (6.51 hours)

The researchers do not suggest that these times, achieved during lab conditions much different from real life, are normal. But the comparison between groups is what's interesting, with the oldest group snoozing about 20 minutes less than the middle-agers, who in turn slept about 23 minutes less than the youngest group. The amount of time spent in deep sleep, measured as "slow-wave sleep," was also less in the older groups.

Video: Video: How do you know if you get enough sleep? Daytime sleepiness was measured by asking the subjects to nap, which count in tallying your overall sleep, said study leader Derk-Jan Dijk, professor of sleep and physiology at the University of Surrey in the U.K. (Previous research has shown that naps are good for you.)

"But, we need to be careful; naps very late in the day may make you feel better for the remainder of the waking day but also disrupt your subsequent night time sleep episode," Dijk told LiveScience.

When participants were asked to lie in bed and try to nap, here's how long it took on average for the members of each group to dose off:

Age 20-30: 8.7 minutes
Age 40-55: 11.7 minutes
Age 66-83: 14.2 minutes

However, the researchers note in the Feb. 1 issue of the journal Sleep that if you're sleepy during the day, then you probably need more sleep.

"Our findings reaffirm the theory that it is not normal for older people to be sleepy during the daytime," Dijk said. "Whether you are young or old, if you are sleepy during the day you either don't get enough sleep or you may suffer from a sleep disorder."

Scientists admit that the role of sleep is not well understood and that they aren't sure how much sleep each person needs. A study last year indicated that some people are genetically programmed to need less sleep.

Video: Simple steps to better sleep Dijk also helped with a 2008 study reported in the journal Current Biology, in which participants stayed in bed for 16 hours in the dark each day for several days, to see how much they would sleep. Younger people slept an average of 9 hours while older people got 7.5 hours.

"The most parsimonious explanation for our results is that older people need less sleep," said Elizabeth Klerman of Brigham and Women's Hospital & Harvard Medical School. "It's also possible that they sleep less even when given the opportunity for more sleep because of age-related changes in the ability to fall asleep and remain asleep."

Again in that 2008 study, however, Dijk and Klerman found that most healthy people, and young people in particular, don't get as much sleep as they need.

If you chronically feel sleepy during the day, Dijk advises you see a doctor. But if you get just six or seven hours of snooze time and feel fine, "then that is OK," he said. But if you think you're okay and find yourself dozing during meetings or nodding off while watching TV, "there still may be a problem."

© 2012 LiveScience.com. All rights reserved.

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