Scientists have discovered a cure for the dreaded Monday morning blues - stop sleeping in on weekends.
A new study has found that lazy Saturday and Sunday lie-ins can disturb your body clock, leaving you fatigued at the start of the week.
Leon Lack, a sleep expert at Flinders University said that people often used the weekend to catch up on the lack of sleep during the week.
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.
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.
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.