The average employee in Wales works an extra 6.2 hours per week - adding up to an extra £735m-worth of value to the Welsh economy.
"That's a huge amount of hours," said Rhianydd Wiliams from Wales TUC.
"We're asking employers to recognise these extra hours and maybe looking at better ways of managing time within the office so that people feel they can leave on time or have a proper lunch break."
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The Welsh Assembly election is less than a week away and the treatment and care of cancer is one of the more contested issues.
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Tonight is expected to be clear and dry - though some parts of Wales will experience frost.