In previous centuries nightmares were believed by some to be caused by supernatural beings. The medical condition sleep paralysis was blamed on witches It was described as being witch-ridden or hagridden – a term referenced by J.K. Rowling for the beloved character Hagrid in Harry Potter The experience is accompanied by a feeling of weight on the chest and hallucinations, which gave rise to the notion of pressing demons such as the incubus and succubus.