Doctors in Pakistan are mystified by the case of two brothers who have come to be known as the "solar kids."

The boys, aged nine and 13, are normal active children during the day. But once the sun goes down, they both lapse into a vegetative state — unable to move or talk. Javed Akram, a professor of medicine at the Pakistan Institute of Medical Sciences, told The Associated Press on Thursday that he had no idea what was causing the symptoms.

The brothers are undergoing extensive medical testing in the capital, Islamabad, and samples of their blood have been sent to overseas specialists for further examination.