The Japanese are well known for their hard work ethic, but a survey suggests over-tired workers are costing the economy $30bn (£16bn) a year.
The study says Japanese workers need more rest
Japan's overworked staff are increasingly struggling to stay awake at work, according to the study by Nihon University School of Medicine.
Other drowsy staff are turning up late or else taking days off for a rest, all leading to less productivity.
Nihon University said it wished to draw attention to the problem.
'Tired not lazy'
"Not everyone who is sleepy at work is lazy," said Makoto Uchiyama, professor and chairman of the department of neuropsychiatry at Nihon University School of Medicine, who conducted the survey.
"It's hard to tell your boss that you are sleepy, but ignoring the problem can lead to losses in the long run."
Japanese workers are renowned for working excessive hours, partly due to the cultural tradition of not leaving work before colleagues, regardless of the actual amount of work.
Seeing shattered workers asleep on crowded commuter trains is a common sight, but as Mr Uchiyama added, you can see a similar picture around the world.
"It may be thought that this is a Japanese problem. But it's not, it's global," he said.
The survey questioned 3,075 workers at a Japanese chemical company.