D´Inferno ink, brush, a kleenex on carboard, 2 coffee, 3 cigarettes.
A bed is supposed to be a warm and comfy place to snooze, surrounded by soft pillows and securely closed doors, safe in the knowledge that the outside world is locked out. When you drop off to sleep, only to wake up with an evil, dark presence standing at the end of the bed, staring at you with ill intentions, the experience can scare the pants off you, especially as you are normally frozen in place, unable to move and run away, almost catatonic with fear.
Demonic presences looming over a sleeping person, or even leaning on their chest or choking them have been part of ghostly lore for centuries. Modern science, though, has come up with evidence that points toward a completely natural explanation that offers some crumb of comfort to those paralysed people lying in bed waiting for the demonic presence to come and kill them in some horrible way. It's called sleep paralysis, and it is an accidental side effect of a generally helpful characteristic of the sleeping body.
Normally, the body switches off movement when it sleeps, to prevent problems like body switches off movement when it sleeps, to prevent problems like sleepwalking, and when a person wakes up, this paralysis turns off. Those threatening faceless presences next to the bed are artifacts created by the brain in sleep mode, and the person only sees and remembers them when he wakes up but is still stuck in paralysis mode, so he cannot move or scream.
A weirdly scientific explanation for sudden feelings of fear and doom is that sound frequencies below the level of human hearing can affect the emotions and produce unexplained feelings of depression, fear and even dizziness. These frequencies can also act on the eyeballs to produce vibration, which in turn can create visual hallucinations of objects like ghostly shadows. Electromagnetic field variations can also interfere with normal ability to sense the environment, contributing to a weird interpretation of the immediate environment. When we measure houses where pervasive haunts occur, the place where the occupants find they can sleep, by trial and error, has the most consistent and normal field strengths. Simple eye disorders like cataracts are also responsible for hallucinations, because the clouded vision of the person affected can trick the mind into seeing things and people that aren't really there. Drug use and psychiatric problems are also potential causes of seeing and feeling things that aren't really there, and even the most cynical person can start jumping at shadows and mistaking noises for footsteps, if the environment is dark and eerie enough.