Workers are being urged to send managers a "bossagram" pointing out they are doing unpaid overtime.
Five million UK workers regularly do unpaid overtime, says the TUC
The idea is to mark this Friday's Work Your Proper Hours Day, being held to highlight the UK's long hours culture.
The Trades Union Congress claims that five million people regularly do unpaid overtime, giving employers £23bn of free work each year.
The union has calculated people doing unpaid overtime are in effect working for nothing until February 25.
The TUC is advising employees to make sure they leave on time on Friday and take a proper lunch break.
It also says those doing free overtime should take some time to reflect on how well they are balancing their life.
TUC general secretary Brendan Barber said: "The bossagram is a fun way to make a serious point.
"Too many workplaces are in the grip of a long hours culture, and with a bit of better organisation people could gain extra hours of free time and still get their work done well."
The TUC also suggests employers should treat workers to a drink or a meal to "show some gratitude" for the amount of unpaid overtime they put in.
UK employees who did unpaid overtime in 2004 would have each earned £4,650 for their efforts if they had received a wage, the TUC reported last month.