For the first time a team from the University of Sydney has tracked people's nightly patterns and found sleep naturally increases and decreases throughout a week.

"The body seems to have a way of adjusting the amount of sleep we require," said the study leader, Chin Moi Chow. ''If you incur a sleep debt, your body will signal a need to catch up on extra sleep.''

Her study found each person had a different sleep cycle, with some taking only a couple of days to catch up, and others taking up to 18 days.

Unlike some previous research, she did not find the participants made up for lost sleep on the weekend. Rather, the 13 young men, whose sleep was measured over a two-week period using a device worn on their arms, made up the sleep at different times, getting up to two hours more sleep on some nights than others.

This suggested the timing of individual cycles was intrinsic, rather than something each person chose, she wrote in the journal Nature and Science of Sleep.

The president of the Australasian Sleep Association, Shantha Rajaratnam, said the study suggested the body had a mechanism for dealing with the amount of sleep we had.

"Over time, it is like you are withdrawing money from the bank, you build up a debt, but eventually you have to pay back that debt. And after that you can start withdrawing again," he said.

But he said it would be "dangerous" for people to assume they no longer needed eight hours' sleep each night.

"Different people need different amounts of sleep but people are not very good at judging the amount of sleep they actually need," he said. "People who think they can get by with very short amounts of sleep tend to compensate for their tiredness by using things like coffee, for example, to keep them alert".

Dr Chow hopes to repeat the research in a larger group, and analyse whether individual patterns match problems linked to sleep deprivation.