The OECD says Japan's sleep deficit is among the worst in the world.

And, according to labour groups, overtime routinely takes Japanese workers above the potentially deadly 100 hours a month mark.

What's more the situation is actually getting worse.

When the survey was conducted in 2014 Japanese on average got 21 minutes more sleep than they do today.

Sleep experts are particularly worried.

They see a labour culture that praises devotion to work over mental and physical health.

At the extreme end of the overtime and sleep deprivation issue is the phenomenon known as karoshi, death by overwork.

Yukimi Takahashi has experienced the dark side of Japan's work culture at first hand.

Her daughter, Matsuri, committed suicide just a year after graduating and joining advertising giant Dentsu.

The country told itself that Matsuri's death must mean something.

That it was time for a change.

In the aftermath Dentsu made a big deal about introducing rules to limit working hours.

Lights automatically switch off on the dot of 10pm, supposedly to stop people from working late, although reports from within the company suggest the toil continues anyway under the glow of desk lamps.

But Japan's overwork crisis isn't just about advertising and media.

It's also about construction.

For the last four years Tokyo has been in an unprecedented construction boom.

But within that boom the biggest project by far has been the preparations for the 2020 Tokyo Olympics.

The problem with all of this is that the construction has been going on at a time where Japan's labour market has never been tighter.

There just aren't enough people to do all this construction work.

And because people are working harder and longer to get everything done on time that's having a terrible effect on the health of those workers.

Karoshi is officially blamed for around 200 deaths every year.

Sleep experts believe that figure understates the problem and that what was assumed to be Japan's paternalistic working culture is in reality a black spot for health.

One of the possible solutions is this place here in Oimachi, right in the heart of Tokyo.

It's a sleep cafe that opened earlier this year as a kind of experiment.

You come here and for $15 you get this bed, blackout curtains, and the chance to sleep for an hour in the middle of the day.

And then at the end you're woken up with a strong cup of coffee and you're back in the workforce.

Sleep expert Dr Jun Kohyama says a nap is fine, but the need for a midday snooze is a signal of a bigger underlying problem.

But there are some who believe there is at least a little scope for change.

Crazy is a small business taking a radically different approach to employee welfare.

Its strategies include paying a cash bonus to staff who sleep longer at night.

Crazy plans bespoke weddings that break with the conservative traditions of the past.

It's a template of innovation that the company is trying to apply to employee welfare.

For Yukimi, nothing can make up for the loss of her daughter.

But if Matsuri's death proves the catalyst behind fundamental change in Japan's working culture at least it would give it some meaning.

As Japan's overworked ranks of blue and white collar workers know all too well, changing the rules is not exactly the same as changing the culture.