Japan's workers are being urged to switch off their laptops, go home early and use what little energy they have left on procreation, in the country's latest attempt to avert demographic disaster.

The drive to persuade employers that their staff would be better off at home with their wives than staying late at the office comes amid warnings from health experts that many couples are simply too tired to have sex.

A recent survey of married couples under 50 found that more than a third had not had sex in the previous month.

Many couples said they didn't have the energy for sex, while others said they found it boring.

A quarter of the men surveyed said they were "too tired" after work, while just under a fifth of women said intercourse was "too troublesome". A study by Durex found that the average couple has sex 45 times a year, less than half the global average of 103 times.

"It's a question of work-life balance," the association's head, Kunio Kitamura, told Reuters. "This is not something that the individual can tackle alone. The people who run companies need to do something about it."

Japan's birth rate, at 1.34 - the average number of children a woman has in her lifetime - is among the lowest in the world and falls well short of the 2.07 children needed to keep the population stable.

If the rate persists, demographers warn that Japan's overall population will drop to 95 million by 2050 from its 2006 peak of 127.7 million.

Earlier this month the crisis prompted Keidanren, Japan's biggest business organisation, to implore its 1,600 member companies to allow married employees to spend more time at home.

Several firms have organised "family weeks" during which employees must get permission to work past 7pm, but most continue to squeeze every last drop of productivity from their staff.

In response, the labour ministry plans to submit a bill early next year exempting employees with children aged under three from overtime and limiting them to six hour days.

The aim is to allow working mothers and fathers to spend more time with their children and, it is hoped, find the time and energy to have more.