But he told the Australasian Sleep Association Conference in Perth that while this might help pay off a "sleep debt", it came at a cost. "We've discovered that these sleep-ins are actually putting your body out of whack enough to change your Sunday night bedtime and set you up for Monday blues," Professor Lack told AAP.

His research team tested the theory by tracking 16 people over a weekend, asking them to go to bed a little later than they would on a weeknight but sleeping-in an extra two hours. By comparing saliva samples and hormone tests he found participants' body clocks had been delayed by 45 minutes. "That might not sound like a lot but it means that you're not quite as sleepy on Sunday night at the normal bedtime and you'll be much sleepier the next day," Prof Lack said.

Questionnaires completed on Monday and Tuesday showed much higher levels of self-reported fatigue and tiredness compared with pre sleep in days. This was because the subjects' circadian rhythms - which determine patterns of alertness and tiredness - had been disturbed, creating an effect similar to jet lag.

By mid-week most people manage to get back on track but then they start staying up later, getting into "debt" once again and perpetuating the cycle. "These days, we're pushing ourselves a lot, particularly during the week and the weekend is our only refuge," Prof Lack said. The problem, he says, is that this comes at a price.

"It's a bit like paying off a mortgage - you take out a big one and you'll have a lot to pay off later on." AAP