Healthy older people can't sleep as much as they used to, according to a new study.

Research in the journal Current Biology looked at people who were asked to stay in bed in the dark for 16 hours for several days. Subjects between the ages of 18 and 32 got about 9 hours of sleep. Those from 60 to 72 got 7.5, a news release said.

Researcher Elizabeth Klerman of Brigham and Women's Hospital and Harvard Medical School said it may show that older people need less sleep, or that they are less able to fall asleep and stay that way.

The news release about the work said that insomnia is a common complaint among older people, but it's not known if the changes are due to social factors, circadian rhythms or other causes.

"While humans can sometimes override the homeostatic set point and not sleep when tired, there is no evidence that they can sleep when they are not tired," Klerman said.

Given the same amount of time in bed, older people take longer to fall asleep and sleep for less time than younger people do.

Most younger subjects slept for many more hours during the study than their usual self-selected sleep times, which could show that they do not usually get enough sleep.