Word of warning: you do not want to search for night hags if you are alone at night. 

Katie first heard about night hags and sleep paralysis from someone she met who experiences sleep paralysis. Dumbfounded by what she heard, she asked question after question and has since been on many internet pursuits to understand the phenomenon a little better. This week, she finally mustered the courage to Google Image search people's depictions of a night hag, which then led to her gaining a greater understanding of sleep paralysis and why it's associated with the supernatural. Sleep paralysis — waking up without being able to move or speak — can provoke feelings ranging from mild anxiety to outright terror. Because hallucinations often occur at the same time as the paralysis, this psychobiological experience feels supernatural to many people.

Further reading:

Why the paralyzing ‘sleep demon’ still haunts humans in the age of science: https://globalnews.ca/news/5177498/sleep-paralysis-demon-science/ 

Understanding ‘Old Hag’ Syndrome: What It Means When You’re Paralyzed in Your Sleep: https://www.healthline.com/health/sleep/old-hag-syndrome