More than a third of married couples in Japan have stopped having sex, many of them because they are too tired or just can't be bothered, a government-backed medical researcher said on Wednesday.

About 37 percent of couples surveyed this year by Kunio Kitamura, head of the Japan Family Planning Association, said they had not had sex for at least a month, compared with 32 percent in 2004.

The most common reason, given by a quarter of the males surveyed, was being too tired after work, while 19 percent of women said sex was too much of a hassle, he told reporters.

"It's a question of work-life balance," Kitamura said in a telephone interview. "This is not something that the individual can tackle¹ alone. The people who run companies need to do something about it."

His written survey involved 647 men and 821 women aged² up to 49, who are married.

The trend could have serious consequences for Japan, whose falling birthrate and aging population are continuing headaches for the government.

The average number of children born to a Japanese woman in her lifetime was 1.34 in 2007, compared with 2.1 in the United States in 2006.

Kitamura said doctors may also be partly to blame for Japan's negative attitudes toward sex during pregnancy³ or after giving birth.

He is set to report to the Ministry⁴ of Health on his findings next year.

 日本一名政府医学研究人员于本周三称，目前日本有超过三分之一的夫妇不过性生活，其中很多人是因为太累或者只是嫌麻烦。

日本计划生育协会会长Kunio Kitamura称，在今年的调查中，约37%的受访者称他们至少有一个月没过性生活，而2004年的这一比例为32%。

他在接受记者采访时说，其中最普遍的原因是下班后太累，有四分之一的男性给出该原因，而19%的受访女性则认为性生活太麻烦。

他在接受电话采访时说：“这是一个工作与生活平衡的问题，无法依靠个人的力量解决。公司管理层应对此采取一些措施。”

这项书面调查共有647名男性和821名女性参加，他们都是年龄在49岁以下的已婚人士。

这一趋势可能会给日本带来严重后果，出生率持续下降及老龄化加重一直是日本政府最头疼的问题。

日本去年的生育率（平均每个妇女所生孩子的数量）为1.34，而2006年美国的生育率为2.1。

Kitamura称，医生的不尽职也是日本对待女性妊娠期和产后性生活问题消极态度的部分原因。

Kitamura将于明年向厚生省提交该调查结果。