Backache is a common cause of absence from work
More than a third of all sick leave is taken on a Monday, research suggests.
The study, by consultant firm Mercer, also found that the highest rate of absence by far occurs in January.
The most common reason for absence was musculo-skeletal problems, followed by viral infections and stress-related illness.
The findings are based on an analysis of sickness management records for 11,000 employees from a range of private sector organisations.
They showed that, during 2008, sick leave in January ran at half a day per person.
Thirteen of the 20 most popular days for sickness absence occurred in January - six of these between 2 and 9 January.
On Thursday 3 and Friday 4 January nearly 5% of the total employee population was absent on sick leave.
The study found that 35% of all sick leave is taken on a Monday, with attendance on the remaining working days becoming higher as the week progresses.
But the lowest sick leave rate was recorded on Fridays - just 3% of the total.
Researcher Phiroze Bilimoria said: "Monday sickness and frequent short-term absences can be a symptom of low employee engagement and morale within certain teams or departments."
Musculo-skeletal problems, such as muscle strains and backache, accounted for 24% of the total number of lost days.
Viral infections and stress-related illness made up 17% of the total each.
Overall, viral infections were the most commonly given reason for absence, but the average period of absence was generally small.
In contrast, few people took time off with stress - but their absences tended to be lengthy.
Female workers took 24% more days off than male workers, and were more than twice as likely to take time off for stress-related illness, exhaustion and depression.
Men were at least twice as likely than women to take time off for muscle sprains, fractures and other physical injuries.
Rest and recuperation
Professor Cary Cooper, an occupational health psychologist at the University of Lancaster, said: "People are doing more work under more demanding conditions so are under more stress and need more rest and recuperation.
"Some employees may feel the weekend is not enough so they take the extra day.
"But I suspect what is happening is people are lying about their reason for sickness absence.
"It is more believable to use back pain as a reason as often stress is seen as a four letter word. It's seen as embarrassing.
"The UK has gone from being a manufacturing economy to a service-based economy in the past 30 years so the problems that are most likely to rise in the workplace are people problems rather than physical problems, despite what the survey suggests."
Professor Cooper said employee audits should be conducted regularly to keep across what was causing problems in the workplace.
"Is it the long hours? Is more flexible working needed?
"Research has shown that, if employers entrust their workers with flexible working, stress-related illness and sickness absence is lower and performance and productivity increases."