Emergency medical teams said that the 35-year-old hiker had survived without food or water after his organs shut down, his pulse slowed and his body temperature fell by a third. They believe that his metabolism all but ground to a halt as he lay on the mountainside, a response that saved him.

“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 emergency unit that treated the man, said. “I believe that his brain capacity has recovered 100 per cent.”

The man was treated for severe hypothermia, multiple organ failure and blood loss caused by a fall, but was not expected to suffer lasting illness.

Mitsutaka Uchikoshi had enjoyed a barbecue with work colleagues in the popular hiking area of Mount Rokko, an 880m (2,887ft) peak near the port of Kobe in western Japan, when he decided against joining the others for the cable-car ride back, opting to walk down on his own.

After losing his way, he slipped, broke his pelvis and then lost consciousness. More than three weeks later he was discovered by a climber. He had almost no pulse and a body temperature of only 22C (72F).

Mr Uchikoshi, a government official from a nearby town, said that his last memory was of enjoying the view as he lay imobilised. “The sun was out, I was in a field, and I felt very comfortable,” he said. “I must have fallen asleep after that.”

Although the autumn nights were chilly, average temperatures hovered around 10C, low enough to cause hypothermia and, eventually, a state resembling hibernation in other mammals.

After almost two months of medical treatment, Mr Uchikoshi finally returned home on Tuesday. “I’m sorry for all the trouble I caused everyone,” he said. “I want to get better quickly and return to work.”

The story struck a chord in Japan, where picnics and barbecues in remote countryside are very popular. But the question now being debated among Japanese doctors is exactly how Mr Uchikoshi managed to survive. After almost a month in the cold, and with only half a bottle of barbecue sauce to keep him alive, hibernation is considered the most likely explanation.

During hibernation, activity in the body’s cells slows to a near standstill, greatly cutting the need for oxygen, and lowering energy consumption.

“This case is revolutionary, if the patient truly survived at such a low body temperature over such a long period of time,” Professor Hirohito Shiomi, a hibernation expert at Fukuyama University in Hiroshima, said. “Researchers would have to clarify whether Mr 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,” he said.

Scientists have long speculated that human hibernation is possible, with potential benefits that include enabling astronauts to undertake longer missions in space. It is also hoped that the hibernation process could be used in medicine to slow cell death when treating fatal conditions such as brain haemorrhaging.

In 2004 a German research team announced the discovery in Madagascar of lemurs that hibernate for many months of the year, in what they claimed was the first proof of hibernation in primates.

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