Three children of a family from a village in the Balochistan province of Pakistan have puzzled medical science because of their "peculiar" dependence on sun and have come to be known as "solar kids" in their village.

The children, aged one, nine and 13, are active and normal like any other children of their age as long as the sun is up. However, once the sun sets, they slip into a vegetative state. Shoaib, Rashid and Ilyas were all born with this strange illness, their father claims.

This is the first reported case of such an illness in the world, Dr Javed Akram, chancellor at the Pakistan Institute of Medical Sciences, told Dawn. Akram said the case is like "a challenging puzzle for medical science to solve", adding that they are researching the condition.

"We took this case as a challenge. Our doctors are doing medical tests to determine why these kids remain active in the day but cannot open their eyes, why they cannot talk or eat when sun goes down," he told the Associated Press.

Akram and his team in now collaborating with an international team of medical professionals to solve the mystery of the "solar kids". Their blood samples have been sent to 13 international collaborators, including Mayo Clinic and John Hopkins Medical Institute in the US, and Guys Hospital in London. Nothing has been found so far despite hundreds of tests. A team is also collecting sand and water samples from Mian Kundi village in a hope of cracking the mystery.

Their father, Mohammad Hashim, said the boys are like any other normal child during the day — wake up early in the morning, go to school, play cricket with friends or their other siblings and even help their father in attending their pet flock of sheep and goats. He said the children are now used to their routines and finish their daily chores before the sun goes down and their disability sets in.

Scientists have ruled out the condition being directly related to sunlight, as when placed in a darkened room during the day, they are able to move around. Hashim, however, is unconvinced. "I think my sons get energy from sun," he said.