Today marks the day when the average worker who does unpaid overtime effectively starts earning for themselves, according to the TUC.
Friday has been designated Work Your Proper Hours Day and the union umbrella body is encouraging staff to take a full lunch break and leave work on time.
Nearly five million people in the UK regularly work for free outside their contracted hours, giving companies an average of £4,955 per year in unpaid overtime, according to the TUC.
Investors in People has warned employers that a long-hours culture is likely to have an adverse impact on the health and productivity of employees and their organisations.
Simon Jones, acting chief executive at the standards body, said: "Just because an employee is working long hours doesn't mean they are being productive. In fact, a long-hours culture can have the opposite effect and breed inefficiency.
"The UK's culture of extended working hours is a real issue that employers need to address or risk the performance of both their employees and the organisation as a whole."