During the day, Abdul Rasheed and Shoaib Ahmed, 9 and 13, are like any other brothers who laugh and play together.
But as soon as the sun goes down they collapse into a vegetative state - completely paralysed and unable to talk.
The brothers, who live in Pakistan, have been dubbed 'the solar kids' as their parents claim they get their energy from the sun.
The case has baffled doctors who have no idea what is causing their symptoms.
Abdul Rasheed and Shoaib Ahmed, 9 and 13, are like any other brothers during the day but collapse into a vegetative state, paralysed and unable to talk, at night
Doctors at a hospital in Islamabad have no idea what is causing the brothers' bizarre symptoms, and are carrying out medical tests
Doctors at a hospital in Islamabad have no idea what is causing the brothers' bizarre symptoms, and are carrying out medical tests
Javed Akram, a professor of medicine at the Pakistan Institute of Medical Sciences, who is treating them, said: '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.
Professor Akram said the Government was providing free medical care to the siblings, who come from an impoverished family.
They are undergoing extensive medical testing and samples of their blood have been sent to overseas specialists for further examination, he said.
Researchers are also collecting soil and air samples from the family's home village.
Their parents say they get their energy from the sun - and so the brothers have been dubbed 'the solar kids'
Dr Javed Akram, left, a professor of medicine at the Pakistan Institute of Medical Sciences, said the case is a 'challenge'. The boys are undergoing blood tests and soil and air samples from the family's home village.
The brothers' father, Mohammad Hashim, comes from a village near Quetta, the capital of southwestern Baluchistan province.
He and his wife are first cousins and two of their six children died at an early age.
Their other two children have not displayed any unusual symptoms.
'I think my sons get energy from sun,' he said.
But doctors have already dismissed the idea that sunlight plays a role, noting that the boys can move during the day even when kept in a dark room or during a rainstorm.
Despite their bizarre affliction, the boys are energetic and cheerful.
'I will become a teacher,' Shoaib Ahmed said, while his younger brother said he wants to be an Islamic scholar.