You awake in the middle of the night to find your body completely paralyzed. As you struggle to breathe, you open your eyes to a frightening sight - a ghostly old woman is sitting on your chest. This experience has been called many things (Old Hag Syndrome, Sleep Paralysis, Sleep Demons, Night Hag, etc.) and has been documented all over the world among various cultures and religions since ancient times. Why does this happen and how is it so universal? Can it be simply explained by biology or could something more nefarious be at play?