A JAPANESE civil servant has described for the first time how he survived for more than three weeks in a mountain forest without food or water in what doctors believe is the first known case of a human in hibernation.

Mitsutaka Uchikoshi went missing on Mount 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," Mr Uchikoshi said shortly before being discharged from a Kobe hospital on Tuesday.

When a climber found him 24 days later, Mr Uchikoshi's body temperature had fallen to just 22 degrees, his pulse was barely discernible and he was suffering from multiple organ failure and blood loss. Doctors who treated him believe he lost consciousness after his fall and his body's survival instincts kicked in, sending him into a state akin to hibernation as the temperature on the mountain dropped as low as 10 degrees.

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