You know your employees work hard... but are they really working the 45-hour weeks their timesheets report? Maybe not: It turns out employees tend to overestimate hours spent at the office--and it could be costing you.

A study conducted by John P. Robinson of the University of Maryland for the Bureau of Labor Statistics found that people tend to overestimate the number of hours they work in a week by 5% to 10%. According to the Harvard Business Review, researchers examined workers’ time diaries and found that workers who say they work 55- to 64-hour weeks are off by an average of 10 hours; people who say they work 65 to 74 hours are overstating by an average of 20 hours. The trend found that the more hours reported, the more workers were overshooting actual hours.

While this information most directly applies to hourly workers who manually report their time, it is also a common practice among full-time workers, researchers say. According to the study, salaried employees tend to overstate the hours they work each week in order to appear proactive and less lazy than their peers.

In related news, the latest employment summary from the Bureau of Labor Statistics showed an increase in the average workweek hours reported for the month of September. Did employees really clock in more hours of work, or did they have a perception problem? You be the judge.