I used to be terrified of sleeping as a teenager. My sister and I both had horrible bouts of sleep paralysis, night terrors and sleep walking when we were 16-20odd.
The whispering that khaos mentions rings true for me, one of the scariest ones that I remember was of a load of voices whispering words I couldn't make out. After a while I could hear them saying "can he hear us?" And the voices were getting agitated. Then one really loud one came close and hissed "He can hear us!" ****ed me up good and proper that did.
A technique I read about which I never managed to use was to wiggle a small toe or or finger. Focus all your energy on moving something on your body and hopefully that will snap you out of it.
I never maintained enough composure to do that and would most often be woken up with a quick slap from my poor twin brother who never once had this kind of shit. Apparently during my night terrors, I'd leap out of my bunk bed and screech like a ****ing eagle while throwing the furniture in our room about. Haha.
I don't believe in any dark meaning behind them. I would put them down to an overactive imagination to be honest. I seems to have grown out of it all now, my wife says I still have my moments of crying out at night but no running around with my arms flailing. I wouldn't recommend sleeping pills or anything by the way, if you can avoid them. Always make me feel like shit the next day.