The woman believed to be the world's oldest person celebrated her 116th birthday on Tuesday.
Kamato Hongo sleeps for two days then stays awake for two days
Kamato Hongo, who lives on the island of Tokunoshima in southern Japan, slept through most of the celebrations to mark her extraordinary achievement, according to her family.
Her grandson Tsuyoshi Karauchi, who lives with her, told the BBC that sleeping was a favourite pastime.
"She will happily sleep two days in a row. She'll wake up for a while, then back to more sleep. We even feed her in her sleep," he told the East Asia Today programme.
Tokunoshima island was also home to a man who reached the age of 120 - the greatest age ever recorded - and there are now more than 20,000 people over the age of 100 in Japan.
The reasons for their unusual longevity could be a diet rich in sea food and low in fat, or Japan's strong welfare system and family support networks.
Mr Karauchi said his grandmother, like a lot of other elderly people, ate miso soup, rice and vegetables.
Asked about the secret of her longevity, he said: "She was brought up in a good environment, ate healthy local food. She survived the war but apart from that it's been a peaceful happy life."
She has never smoked, he said, "but she did start drinking about 20 years ago in her 90s".
"She enjoys a glass of sake when she's awake now. Almost every day," he said.
Mrs Hongo, who is part of a five-generation family, is sociable despite her age, Mr Karauchi said.
"When guests come round she always offers them a cup of tea. She likes having people in the house," he said.
But there is a downside in Japan to this otherwise rosy picture of health.
Japan is an aging society with a shrinking workforce, which is supporting more and more retired people.
Economists are predicting a crisis in the state pension system within a few years, because the benefits being paid out far exceed payments being made into it.
On the other hand, Japan's elderly are generally fitter and more active than in other countries.
The oldest man and woman ever to climb Mount Everest are both Japanese.