Employees in the UK are being urged to send their manager an anonymous "bossagram" pointing out how much unpaid overtime they work.
The TUC launched the idea to mark Work Your Proper Hours Day on Friday 25 February, which is being held to highlight the UK's long hours culture.
The TUC has calculated that people working unpaid overtime are, in effect, working for nothing. 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 better organisation people could gain extra hours of free time and still get their work done well."
Managers are being encouraged to make sure their staff leave on time on Friday - and take a proper lunch break.
The TUC even suggests that employers should treat workers to a drink or a meal to "show some gratitude" for the amount of unpaid overtime they put in.