A young journalist's gruelling work schedule - including a single month with 159 hours of overtime and just two days off - triggered the heart failure that killed her at 31, Japanese labor regulators ruled.

Authorities officially attributed Miwa Sado's death to "karoshi" - the Japanese word for a death due to overwork - according to information released this week by NHK, the public broadcaster that employed her.

Sado, a political reporter, had been covering elections for Tokyo's government, and Parliament, in the months leading up to her death in 2013. She died three days after the elections for Japan's upper house.

NHK had not released information that regulators had compiled about the death until this week.

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The determination that Sado's death was caused by overworking has brought renewed scrutiny to the working culture in Japan, where hundreds, if not thousands of people are believed to work themselves to death every year.

One official with the public broadcaster told reporters that her death was indicative of "problem for our organisation as a whole, including the labour system and how elections are covered".

Japan's working culture, where long hours and after-work social engagements are typical, dates back decades, as the Post's Anna Fifield has reported:

"It began in the 1970s, when wages were relatively low and employees wanted to maximise their earnings. It continued through the boom years of the 1980s, when Japan became the world's second-largest economy and everyone was on the juggernaut.

"And it remained after the bubble burst in the late 1990s, when companies began restructuring and employees stayed at work to try to ensure they weren't laid off.

"Still, irregular workers - who worked without benefits or job security - were brought in, making the regular workers toil even harder.

"Now, no one blinks an eyelid at 12-hour-plus days."

"In a Japanese workplace, overtime work is always there. It's almost as if it is part of scheduled working hours," said Koji Morioka, an emeritus professor at Kansai University who is on a committee of experts advising the government on ways to combat karoshi.

"It's not forced by anyone, but workers feel it like it's compulsory."

The country classified 189 deaths from overwork in 2015 - 93 suicides and 96 from heart attacks, strokes and other illnesses related to overwork - though experts believe the actual number may be much higher.

In addition to long hours, holidays routinely go unused: On average, employees used less than half of their leave time in 2015 - about nine days a year, according to the Guardian.

A study of 10,000 companies in Japan released last year found that more than one-fifth exceeded an 80-hour per month overtime threshold. More than one in five Japanese workers work 49 hours or more each week, compared to 16.4 per cent in the United States, 12.5 per cent in Britain and 10.1 percent in Germany, according to the Associated Press.

In Sado's case, 159 hours of overtime averages to more than 5.5 hours a day over the course of a 28-day month. According to the Asahi Shimbun, Sado was busy covering candidates and their supporters, shooting footage of speeches and attending meetings during the election.

"She was under circumstances that she could not secure enough days off due to responsibilities that required her to stay up very late," said a release from labour regulators, according to the Shimbun. "It can be inferred that she was in a state of accumulated fatigue and chronic sleep deprivation."

Sado started work at the NHK in 2005, when she was in her early 20s, according to the Japan Times. NHK said that it waited to make information about her death public out of deference to her family, reports said.

"Even today, four years after, we cannot accept our daughter's death as a reality," Sado's parents said in a statement released by the broadcaster to Japanese media. "We hope that the sorrow of the bereaved family will never be wasted."

The deaths of other young Japanese workers have brought renewed attention to the issue in recent years.

In 2015, the despondent messages left on Twitter by Matsuri Takahashi, a 24-year-old who was working more than 100 hours of overtime a month at an ad agency, drew wide attention after she killed herself by jumping from a company dormitory.

"I'm going to die. I'm so tired," she wrote in one message.

The company's president and chief executive later resigned, due in part to outcry over her death.

Later that year, 34-year-old maintenance worker Kiyotaka Seriwaza killed himself after putting in 90-hour weeks at a company from which he had tried, unsuccessfully, to resign.

The Government has been taken steps to change the culture around work to address the problem of karoshi, passing legislation with the goal of reducing the amount of workers working more than 60 hours a week and working to entice employees to use their paid vacation time.

Early this year, a Government spokesman told Bloomberg News that Japan needs to "end the norm of long working hours so people can balance their lives with things like raising a child or taking care of the elderly".

Companies have been joining the effort, taking steps to encourage workers to actually leave work, use their vacation days, and spend more time away from work.

Dentsu has begun switching off the lights in its headquarters at 10pm, and now requires workers take at least five days off every six months. Japan Post Insurance, a life insurance company, switches off its lights at 7.30pm Yahoo Japan has been considering a four-day workweek.

Sado had sent an email in the weeks before her death that warned of the toll her work was taking on her, the Shimbun reported.

"I am too busy and stressed out and think about quitting my job at least once a day, but I guess I have to hang on," she wrote, according to the newspaper.

She was discovered in her bed, holding her cellphone in her hand.