Exploding head syndrome is a 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, roar or a ringing noise.
This usually occurs within an hour or two of falling asleep, but is not the result of a dream and can happen during the day as well.
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 fear and anxiety after an attack, accompanied by elevated heart rate. Attacks are also often accompanied by perceived flashes of light (when perceived on their own, known as a "visual sleep start") or difficulty in breathing.
The condition is also known as "auditory sleep starts." It is not thought to be dangerous, although it is sometimes distressing to experience.
Note that exploding head syndrome does not involve the head actually exploding.
The cause of exploding head syndrome is not known, though some physicians have reported a correlation with stress or extreme fatigue. The condition may develop at any time during life and women are slightly more likely to suffer from it than men. Attacks can be one-time events, or can recur.
The mechanism is also not known, though possibilities have been suggested; one is that it may be the result of a sudden movement of a middle ear component or of the eustachian tube, another is that it may be the result of a form of minor seizure in the temporal lobe where the nerve cells for hearing are located.
Electroencephalograms recorded during actual attacks show unusual activity only in some sufferers, and have ruled out epileptic seizures as a cause.