The Japanese spent more than they saved in the 12 months ended March 2014, the first time that's happened since the data set began in 1955, with the savings rate at a negative 1.3 percent in the last fiscal year.
"It's something to keep an eye out for in the medium-term because Japan's debt has been funded domestically, and very cheaply. But foreign investors would require a more appropriate risk premium," said Toru Yamamoto, Daiwa' Securities chief rates strategist.
Japan has quite a bit of debt, with the country's debt-to-gross domestic product (GDP) at over 220 percent, one of the highest in the world, financed by the domestic savers and Japanese government bond (JGB) investors at some of the lowest interest rates globally.
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On Thursday, the yield on 10-year JGBs fell to 0.305 percent, a record low. By contrast, the historical low for Germany's 10-year yield is 0.565 percent, hit earlier this month, with the country's debt-to-GDP ratio at around 80 percent. The U.S. 10-year Treasury yield is around 2.26 percent, with an around 106 percent debt-to-GDP ratio.
Still, "for now we can't see where the floor is for government bond yields," said Yamamoto.The JGB markets are a flush with money provided by the Bank of Japan's quantitative easing program, he said, adding everyone is buying some JGBs because they need to roll over redemptions. "It all adds up and pushes yields down yet further," he said.
On October 31, the central bank surprised the markets by expanding its annual JGB purchases to 80 trillion yen, from 50 trillion yen, and extended the duration of bonds it holds to about 7-10 years.