If you struggled to get out of bed this morning, scientists have come up with a handy excuse - you may be suffering from social jet lag.
They say sleeping in at the weekend can leave us too tired for the start of the working week, with many remaining groggy until Wednesday.
The immediate effects include poorer memory and reaction times, which would explain that familiar Monday morning feeling of sluggishness.
Over time, repeated changes to sleep patterns can also make you fatter and more likely to turn to cigarettes and caffeine, research shows.
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The study at Rush University in Chicago into how changes in sleeping patterns affect reaction times suggests that a shift of just two hours can leave you worse off in the week.
Test subjects were asked to hit a button when they saw a bullseye appear on a screen, the journal Applied Ergonomics reports. 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.
Lead researcher Helen Burgess said the problem arises when, having shifted their body clocks with a couple of lie-ins, many face a 'rude awakening' with an earlier start for work on Monday morning.
- Daily Mail