UK workers do £4,800 worth of unpaid overtime on average every year, research has indicated.
Four million people in the UK work more than 48 hours a week
Britons work for nothing for an average of seven hours and six minutes each week, according to a TUC survey.
If staff did all of this work at the start of the year, they would not get paid until February 23, says the TUC.
It has designated that date as Work Your Proper Hours Day, and is urging people to take their breaks and go home on time to mark it.
North-east England: Seven hours, 42 minutes
London: Seven hours, 36 minutes
Eastern England, West Midlands, Yorkshire and the Humber, Northern Ireland: Seven hours, 12 minutes
Scotland: Six hours, 30 minutes
Employers are also being urged to take the opportunity to thank their staff and even buy them lunch or an after-hours drink.
Workers in north-east England did the most unpaid overtime last year, an average of seven hours and 42 minutes a week.
Staff in Scotland clocked up the lowest amount of overtime - six hours and 30 minutes.
TUC General Secretary Brendan Barber said: "We work the longest hours in Europe and too many workplaces are gripped by a long-hours culture.
"We do not want to turn Britain into a nation of clock-watchers and few mind putting in extra effort from time to time when it is needed, but it is too easy for extra time to get taken for granted and then expected every week."
The research showed that when all workers' unpaid hours were totalled, UK staff were saving their employers £23bn a year.
The TUC said that working long hours was "a very real hazard".
It warned that people who did too much were putting themselves at risk of injury, heart disease, stress, depression, serious headaches and bowel problems.
The Health and Safety Executive has calculated that sick leave costs the UK economy £12bn a year.
AVERAGE WORKING HOURS
In 2004/5, almost 13 million working days were lost to stress, depression and anxiety, according to health officials.
But UK workers seem to show little sign of easing up.
TUC research shows that four million of them are working more than 48 hours a week on average - 700,000 more people than in 1992.