why nightmares exist ??

I’m sure that everyone had a nightmare in there life .Kids , teenagers ,adults , elderly all of us we can have a nightmare but why ?! why do we have nightmares?? why do we see them and sometimes our nightmares looks the same even if we where in different places .

there are a-lot of  people who beliefs that the demon is the reason for it ..

 

also Dreams have meanings and nightmares too . 

there are few common nightmares.

•  Teeth Nightmares:
• 
• Falling nightmares
• 

but have you ever so a shadow standing next to the door or in your bed room staring at you and you see it but you cant move ?? i have see it alot and i know people who sow the same thing and after i did some  research  i fine alot of  blogs and even movies about it ..

but why ?? is it real thing or our brain is making this image 

if you want to know more about it go here  https://tothestars.media/blogs/news/39171521-aliens-shadow-men-demons-the-nightmare-of-sleep-paralysis