If getting out of your bed on a Monday morning seems like a Herculean task, then you could be suffering from social jet lag, according to scientists.
The immediate effects include poorer memory and reaction times, explaining a Monday morning feeling of sluggishness.
A shift of just two hours can leave you worse off in the week, says a study by Rush University in Chicago into how changes in sleeping patterns affect reaction times.
The phrase 'social jet-lag' was coined by German researcher Till Roenneberg, who has shown that the phenomenon can take a hefty toll on health, by raising the odds of drinking, smoking and relying on caffeinated drinks, the journal Applied Ergonomics reported.
While travel-induced jet lag is something most of us only endure occasionally, social jet-lag can make itself felt every week, according to the Daily Mail.
Over time, repeated changes to sleep patterns can also make you fatter and more likely to turn to cigarettes and caffeine, the research showed.
Test subjects were asked to hit a button when they saw a bulls eye appear on a screen.
Unsurprisingly, they were slower in the mornings than in the evenings - but they were also far slower after a pattern of sleep similar to getting up early on a Monday morning after a weekend of late starts.
Helen Burgess, from Rush, who led the study, said: "The weekend sleep in is just the process by which people shift their clocks later - it wouldn't be a problem except for that rude awakening on Monday morning when all of a sudden we need to shift earlier."
Her latest study, of 65,000 men and women, found that those with different weekday and weekend sleeping schedules were more than three times as likely to be overweight as those who tended to keep similar hours day after day.