Exploding head syndrome: an unusual form of parasomnia (type of sleep disturbance) in which a person perceives loud noises (such as a gunshot, explosion, or crash) when awakening or falling asleep. Some estimates suggest that up to 10% of the population may have experienced this phenomenon. The perception of noise is often associated with fear, racing heartbeat, palpitations, and confusion. There is no associated pain. The cause of exploding head syndrome is poorly understood.