How often do you get a really good night's sleep? We all know how irritable, distracted and clumsy we can be when we don't get enough slumber. A report published in Current Biology suggests that although we need less sleep as we get older, people of all ages may be habitually "underslept". Professor Elizabeth Klerman, who led the study at Harvard Medical School, claims that this has implications both for normal sleepers and for the treatment of insomnia.

Together with Professor Derk-Jan Dijk at the University of Surrey, Klerman looked at sleeping patterns of old (60-80 years) and young (18-30) people. If they were forced to have 16 hours of bedrest (12 hours at night and 4 in the afternoon), the older group slept for an average of 7.4 hours while the youngsters slept for 9.2 hours.

Everyday experience might suggest that the older folk would make up by having the odd impromptu nap during the day. However, the researchers found that, when asked to fall asleep during the day, the senior team were less likely to drop off and, if they did, took more time to do so than their junior counterparts. The results are consistent with an age-related reduced need for sleep, although they could indicate increased difficulty in getting to sleep as we get older, admits Klerman.

The scientists also said that many of us, young people in particular, may not get enough sleep in our everyday lives - we carry a 'sleep debt'. On the first day of extended opportunity, people snoozed for longer than they usually would, while the following day they showed the positive effects of a good night's sleep. On subsequent days, they spent less time asleep having paid off the sleep debt, according to Klerman and Dijk.

The participants in the study slept in a dark, sound-proof room without access to a clock. The night before their extended sleep opportunities, they dozed for as long as they usually would. As they were healthy, taking no medication, with normal sleep patterns, Klerman admits that the results can't necessarily be generalised to insomniacs, but points out that clinicians and scientists have to find out how much sleep people of various ages need before they can say whether or not they are getting enough.

According to the report, the reduction in sleep with age included equal amounts of Rapid Eye Movement (REM) sleep, associated with dreaming, and non-REM sleep, which includes deep sleep.