I started reading about the folklore and history of sleep paralysis and nightmares. The phenomenon exists in all cultures and everywhere has a different explanation of it. However, a common motif is the idea that there is someone or something pushing on your chest so that you can’t move. In old Germanic cultures it was believed that nightmares rode in on a horse (hence the name). In Slavic culture it was believed these spirits often entered through your doors keyhole. In Nordic cultures, they often would tangle your hair and the world around you causing the paralyzed feeling.

The word nightmare actually originates from the old English word mære - which referred to the spirit that entered your room while you slept to entangle you in dark dreams.