Mitsutaka Uchikoshi went missing on Mt Rokko in western Japan on October 7 after a barbecue with colleagues. Rather than joining them for the return trip by cable car, the 35-year-old decided to walk down the mountain, but lost his way, slipped in a stream and broke his pelvis.

"On the second day, the sun was out, I was in a field, and I felt very comfortable. That's my last memory," he said, shortly before being discharged from Kobe city general hospital on Tuesday. "I must have fallen asleep after that."

When a passing climber found him 24 days later, Mr Uchikoshi's body temperature had fallen to just 22C (72F), he had a barely discernable pulse and he was suffering from multiple organ failure and blood loss.

Doctors who treated Mr Uchikoshi believe he lost consciousness after his fall and that his body's natural survival instincts kicked in, sending him into a state akin to hibernation as the temperature on the mountain dropped as low as 10C.

"He fell into a state similar to hibernation and many of his organs slowed, but his brain was protected," Dr Shinichi Sato, head of the hospital's emergency unit, told reporters. "I believe his brain capacity has recovered 100%."

Doctors said they did not expect him to experience any lasting ill-effects.

Mr Uchikoshi said he could not remember anything after the second day of his ordeal on the mountain, a popular spot for hikers and picnickers. One report that emerged while he was still in hospital said he had sipped bottled water and barbecue sauce before falling unconscious.

Experts say it remains unclear how Mr Uchikoshi managed his extraordinary feat of survival with his metabolism almost at a standstill.

"This case is revolutionary if the patient truly survived at such a low body temperature over such a long period of time," Hirohito Shiomi, a professor at Fukuyama University, told the Associated Press.

"Researchers would have to clarify whether Uchikoshi's body temperature dropped very quickly, or whether he started losing body heat much later and was in fact dying when rescuers found him."

Mike Grocott, an intensive care specialist at University College London, said: "People can be profoundly hypothermic and survive for moderate periods of time [but] this sounds like an utterly extraordinary case. I haven't heard of one similar with anything like the same length of survival."

However, he added: "How they can say that he'll be completely unharmed I'm not sure. I would be more circumspect about the neurological and general outcome."

Scientists have long said human hibernation is theoretically possible, and could potentially be put to use to slow cell death when treating brain haemorrhaging and other fatal conditions.

In 2001, Canadian toddler Erika Nordby wandered outside at night in sub-zero conditions and was later found by her mother, almost frozen solid. Despite the fact that she was pronounced clinically dead - her heart had stopped beating for two hours and her temperature had dropped to 16C from the normal 37C - Erika made a full recovery.

Deep sleep

Hibernation is widespread in many animals and involves slowing down the activity of cells to almost a standstill, reducing the amount of oxygen needed to survive.

Humans do not hibernate naturally but, technically, it is not impossible: doctors regularly cool people to around 20C during heart surgery but this is done for a few hours at most. Even then patients can suffer subtle neurological problems. "Whether it leaves you completely normal is less clear," said Dr Grocott.

In 2004, a German research team said lemurs had been discovered in Madagascar that hibernate for many months of the year, offering what they claimed was the first proof of hibernation in primates.

Last year scientists in America used hydrogen sulphide gas to induce hibernation in laboratory mice. This chemical occurs naturally in mammals and researchers said if its use could be adapted for this purpose in humans, it would prove useful for surgery and keeping organs viable for transfer.