As strange as the name sounds, exploding head syndrome is actually a rare and relatively undocumented sleep phenomenon. While sleeping or dozing, a person with the condition hears a terrifically loud sound in their head, such as a bomb exploding, a clash of cymbals or a gun going off.
“It’s usually described as a loud bang or pop that occurs in the first third of the night,” says Dr. Neil Kline, sleep physician and representative of the American Sleep Association in Wilmington, Del. “It’s a sensory phenomenon. The individual senses that some type of explosion has occurred nearby, but ultimately realizes it’s in their head. It’s not associated with pain or with any disorder that we know of and there are no physiological medical consequences that are associated with it.”
(via William Gibson)