Have a bad case of the Mondays? More than one-third of all sick days are taken on Mondays, according to a new study from consultant firm Mercer, which also found that more absences occur in January than any other month. Musculo-skeletal problems are the most common reason for days away, followed by viral infection and stress-related illness, the BBC reports. The study, based on an analysis of records from 11,000 private sector employees, showed that, in 2008, sick leave in January totalled one half-day per person: 13 of the 20 most popular sick days were in January, six of them between Jan. 2 and 9. Meanwhile, 35 per cent of all sick leave is taken on a Monday, and the lowest rate is on Friday, at just 3 per cent of the total. Female workers took off 24 per cent more days than male workers, and were twice more likely to suffer from stress-related illness, exhaustion and depression.