The sensation of hearing a very loud noise in the middle of the night, typically when half-conscious. However the sound never actually happened. Sounds range from explosions and gunshots to even human noises. Rarely, there is a visual phenomenon accompanying the noise, such as a flash of light. The exact cause of the syndrome is unknown.
I woke up at 5am from a loud crashing sound, but I think it was just Exploding Head Syndrome!