British businesses are being urged to encourage staff to take regular breaks and eat well to combat a "siesta syndrome" costing millions a year.
Eating fast food at lunchtime may cause an afternoon slump
A survey of 1,000 people by workplace catering firm Avenance suggested three out of four workers said they were less productive in the afternoon.
Tiredness, boredom and hunger were employees' main reasons for working less efficiently after 2pm.
Regular breaks and healthier food could help overturn it, Avenance said.
"Not only has our research shown that workers are significantly less productive during the afternoon but the majority admit to making mistakes during this time," said Avenance chief executive Mike Audis.
"Less work and costly errors means the UK siesta syndrome is having a devastating impact on a company's bottom line and its reputation."
More than half of the survey's 1,000 respondents said they made mistakes in the afternoon.
The survey showed productivity peaked at 12pm, then suffered an "alarming drop".
"Many companies already subsidise gym membership to improve the well being of their workforce but there are some other, simple steps employers should consider to combat siesta syndrome," said Mr Audis.
Regular morning, lunch and tea breaks, as well as "eating healthy, well-balanced food throughout the day" would help stop the syndrome, he said.