Each night, two brothers in Pakistan suddenly become unable to talk, move, eat or drink.
After the sun goes down, they lie in a vegetative state. During the day, they are normal, playful boys.
Doctors in the capital of Islamabad, where the boys are being studied, say they have no clue as to what causes the children's mystifying behavior.
"We took this as a challenge," said Javed Akram, a medicine professor at the Pakistan Institute of Medical Sciences, according to The Associated Press.
"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 the sun goes down," he said while visiting the boys in the hospital.
Abdul Rasheed, 9 and his brother Shoaib Ahmed, 13. (A.P./B.K. Bangash)
The government is providing free medical care to the brothers, he said.
They belong to a poor family in a small village near Quetta. Soil and air samples have been gathered from their neighborhood. Blood samples have been collected from the boys. All will be sent overseas to be examined by specialists.
The father, Mohammad Hashim, has a simple explanation. "I think my sons get energy from the sun," he said.
The boys' parents are first cousins. Of their six children, two died at a young age. The other two children do not share the boys' symptoms.
Brothers Abdul Rasheed, 9, and Soaib Ahmed, 13, at a hospital in Islamabad (AP/B.K. Bangash)
Doctors don't think sunlight has anything to do with the brothers' ailment. They moved around fine during the day even if they are in a dark room, physicians said.
The boys seemed cheerful and active during a recent daytime trip to a canteen for tea. Shoaib Ahmed told The AP he wants to be a teacher when he grows up. His younger brother said he wants to become an Islamic scholar.