Exploding head syndrome is a rare condition first reported by a British physician in 1988 that causes the sufferer to occasionally experience a tremendously loud noise as if from within his or her own head, usually described as an explosion or a roar. This usually occurs within an hour or two of falling asleep, but is not the result of a dream. Although perceived as tremendously loud, the noise is usually not accompanied by pain. Attacks appear to increase and decrease in frequency over time, with several attacks occurring in a space of days or weeks followed by months of remission. Sufferers often feel a sense of terror and anxiety after an attack, accompanied by elevated heart rate. Attacks are also often accompanied by perceived flashes of light or difficulty in breathing. The condition is also known as 'auditory sleep starts'. The noise may be accompanied by a perceived bright flash of light, and the light on its own is known as a 'visual sleep start'.
Note that exploding head syndrome does not actually cause one's head to explode.