Nightmares are the a halmark of certain pscychological disorders, like PTSD. Often, these nightmares are treated with medications that produce deep, dreamless sleep and thus erradicate the nightmare.
Long before modern psychology or Freud, people suffered from nightmares. They awoke screaming or with feelings of something crushing them or holding them to the bed. In most cultures, these things were blamed on demons and bad spirits and many still blame such symptoms on bad spirits. Historically some of the better-known spirits of this sort are; Greek ephialtes (one who leaps upon) and mora (the night "mare" or monster, ogre, spirit, etc.), Roman incubus (one who presses or crushes), German mar/mare, nachtmahr, Hexendrücken (witch pressing), and Alpdruck (elf pressure); Czech muera, Polish zmora, Russian Kikimora, French cauchmar (trampling ogre), Old English maere (mab, mair, mare-hag), hagge, (evil spirit or the night-mare--also hegge, haegtesse, haehtisse, haegte); Old Norse mara, Old Irish mar/more, Newfoundland Ag Rog (Old Hag), and the Spanish pesadilla. All of these evil spirits entered dreams and possessed the mind and body of the sleeper.
As I talk to many people who have been tormented by more malevolent hauntings, they have described symptoms of crushings and pressure while they were asleep that was commonly associated with many of these bad spirits. The people I interviewed believe that these symptoms were caused by demons or bad spirits. We know many bad dreams are what psychologists say they are. They are anxiety and life stress seeping over into our resting minds. However, is it possible that these historic interpretations of nightmares aren't all wrong? Is it possible that some nightmares are caused by these old demons creeping up into our beds and pressing down on us while we sleep?