It's not really a nightmare but I sometimes get sleep paralysis. To simplify, it's when your head wakes up but your body doesn't.
The first two times it happened I was I woke up lying flat on my back completely unable to move (well I could wiggle a little). This was disconcerting enough, but it was accompanied with a very intense sensation that someone or something was in my bedroom and they had very evil intent and I felt as though there was a great weight pressing down on me. So I was lying there thinking to my self, 'Oh shit...there's something there!', but I had no idea what. I just 'knew' it was something evil and I had to get up and confront it. The trouble was I wasn't able to move. So I lay there flicking my eyes around the room looking for whatever it was. After what seemed like hours I said to myself, 'Ok, Tom. Count to 10 and sit up'. I counted to 10 and used all my might and forced myself to sit up. As soon as I was sitting up in bed I felt absolutely fine and a little bit confused.
It wasn't until it happened a few more times that I wondered what it was and found out about sleep paralysis. It's bloody weird.
It happened to me again this Monday. As I was falling asleep I noticed that slightly too familiar feeling that something was wrong. I was sort of lying on my front and got the feeling of evil presence. I then felt as though something (it strangely enough felt like a large cat or dog) got up onto my bed. I can only describe it as a physical hallucination. I felt the weight shifting around on my bed and it worried the crap out of me. Luckily I snapped out of it pretty quick. The bugger was it happened again a few minutes later as I started dosing off again. That really messed with my nights sleep.
Other times I have hallucinated while it happens. I once saw a 'monster' on the end of my ape. Like a fat hairy ape/human creature with a weird brown head. In this case it wasn't very scary because the monster seemed rather pathetic (it was wheezing heavily). Another time I saw a young chil standing at the end of my bed staring at me in a very demonic fashion. That was quite offputting. So much so that I attempted to kick the child away but could only manage a slight leg movement which luckily was enough to wake me up and snap out of it.
Sometimes sleep paralysis is entertaining but a lot of the time when it happens it is extremely scary. Until you snap out of it. Then suddenly you feel normal again and a little silly for getting so worked up about it.
The odd thing is that it only really started happening over the last few years. Last Monday was the first time it happened in ages so I expect I'm getting over it now! Woop woop!