So sometimes if I stay up really late, or drink lots of coffee, I sometimes get a strange feeling that some sort of generic “monster” is stalking me somehow. It sometimes happens when I’m playing a video game as well, even if the game is 100% NOT a horror game, I get really paranoid that something will suddenly pop out at me for no reason. Why does this sometimes happen?
My WAG would be mental illness.
Since we are not able to make a remote diagnosis of the OP’s mental state, let’s move this to IMHO.
Colibri
General Questions Moderator
Perhaps there is a clue somewhere in the amount of time you spend deliberating the reading and viewing material that you describe below?
A squad of soldiers can’t save you now.
Probably just anxiety? I was terrified of the dark as a child and it took a long time to get over it. When I was in my 20’s I remember sometimes being scared that something would grab my shoulders while I was using the computer, or that something under the bed would grab my ankles. Now that I’m in my 30’s the only thing I sometimes get scared of is the bathroom mirror, I worry that my mirror image is going to do something weird (I’ve peed in bottles to avoid seeing the mirror at night).
You sure it’s coffee and not meth? Take a nap every few days. That’ll ease the paranoia. Or, you know, cut back.
I’m a full-grown adult, and I still get “haunts” in the darkness. I think it may be how our brains are hard-wired to be wary of threats.
Stuff and nonsense, the OP has to man up.
Of course, to this day I feel uneasy at night if the closet door is open, because of what may lurk in there. But that’s just common sense.
That’s why a nightlight is essential. It keeps away all of those things which go bump in the night which are frightened by a low wattage light bulb.
It’s me. I have that effect on people. If I get too close, they flee in terror. I’m sorry. 
It’s not paranoia is there are monsters that are out to get you.
Based on your previous threads, I’d say mental illness.
Perhaps the OP can peruse this compilation of evil laughs? Odds to bodkins his monster is amongst this collection of mad villians.
Not necessarily. I get the same feeling during every political campaign. 
I would certainly avoid looking under the bed or in the closet. And definitely sleep with the lights on. Monsters hate lights. Wouldn’t hurt to keep your head under the covers, just in case.
OhMyGod, what if it turned out to be an evil blimp?? I’d run screaming from the house!
Well, all except Mothra.
I think we are definitely hard wired to be scared of the dark. I know I still have issues looking out a window at night [something might be looking back at me?] which when I was living on the 6th floor of a condo building and my view was of the Atlantic Ocean was a bit silly. [Though I do admit I hate having my bedroom on the ground floor as windows really aren’t secure unless it is break resistant ‘glass’. One thing i learned working in the security industry is that buildings unless made impervious are really not much better than a canvas tent If someone wants in, they will get in.]
I know I have a need for sound when I sleep otherwise I am still hyperalert [thanks, PTSD.] so I sleep to a long rain and thunder sound that blacks out my screen so the light doesn’t interfere.
My advice to the OP: read a short story by Dr. M. R. James every night by candlelight before going to sleep.