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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It will remain breezy with further blustery and at times thundery showers.
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Showers becoming increasingly wintry even to lower levels, bringing an ice risk.