Exploding head syndrome (EHS) is a form of hypnagogic auditory hallucination and is a rare and relatively undocumented parasomnia event in which the subject experiences a loud bang in their head similar to a bomb exploding, a gun going off, a clash of cymbals, ringing, an earthquake, or any other form of loud, indecipherable noise that seems to originate from inside the head. This noise usually happens at the onset of sleep or within an hour or two of falling asleep, but is not necessarily the result of a dream.