Mina Mori was a fun-loving 26-year-old, recall her parents. But a few weeks after she began working full time they say she became depressed, irritable and unhappy. In June 2008 she jumped to her death.

In the previous month she had worked 121 hours overtime, well above the Japanese government limit beyond which death from overwork – known as karoshi – becomes more likely. “My daughter was worked to death,” says her father.

Mori’s suicide is the source of a civil lawsuit between her parents and former employer, the restaurant chain Watami. The labour standards office accepts that her death qualified as karoshi but Watami disputes that finding.

Since the government broadened the definition of karoshi in the 1990s to include suicide from overwork, dozens of similar disputes have emerged. Employers will fight tooth and nail to avoid losing.

The karoshi phenomenon suggests one cliche about Japan – that it is a nation of exhausted workaholics – is true. But on paper at least, the country is on a par with its main industrial competitors.

Japanese workers are entitled to about 18 days paid holiday a year – close to the global average of 20, according to Harris Interactive, a leading market research firm. They also enjoy 16 national one-day holidays.

Reluctant leave-takers In reality, however, employees in Japan are among the least likely in the world to use their paid holidays. Last year they took, on average, just seven days off.

Moreover, full-time Japanese employees work exceptionally long hours, officially clocking in for 400 hours a year longer than their counterparts in Germany or France.

Japan’s addiction to work becomes more striking when you look beyond the official statistics. Japanese employees work millions of hours every year without pay.

Impact on family life Largely unrecorded, this overtime is notoriously difficult to quantify. But some say research suggests it affects more than half of all full-time employees.

Suicides are only the most extreme expression of the overwork phenomenon. A 2013 health ministry survey found that a fifth of young fathers in Japan spend just an hour with their children every day. The same survey claims that less than 3 per cent of Japanese men take paternity leave.

Alarmed, Japan’s Diet is deliberating a so-called karoshi Bill that will make the government responsible for preventing deaths from overwork. The parliamentary dominance of the ruling Liberal Democratic Party (LDP) means the Bill is expected to pass next month.

But many doubt it will work. “The social norm in Japan expects workers to prioritise work responsibilities over family and community obligations,” says Rika Morioka, a public health specialist. The government’s heart is not in the proposed reforms, she adds.

As evidence, campaigners point to the growth of so-called “black companies” – modern sweatshops where 80 hours unpaid overtime a month is common.

Watami consistently tops the critics’ list of black companies in Japan.

It’s founder, Miki Watanabe, is now a politician with the LDP and one of the country’s wealthiest lawmakers.