A new study published in Current Biology found that healthy older people not only need less sleep, but actually lose some ability to sleep, compared to younger people.

“The most parsimonious explanation for our results is that older people need less sleep,” says Elizabeth Klerman of Brigham and Women’s Hospital and Harvard Medical School, Boston. “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.”

Instead of investigating the sleep patterns of older people, the new study investigated the actual capacity of sleep for older people (age 60 to 72) compared to young people (age 18 to 32). When participants were asked to stay in bed for 16 hours in the dark each day for several days, younger people had an average of 9 hours of sleep compared to 7.5 hours of sleep for older people.

The researchers note that the results of the study apply only to healthy individuals taking no medication and having no medical conditions or sleep disorders.

“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,” says Klerman.