It happens to me quite frequently. Far more often than most. Some really crazy stuff has happened during these things.
The first few times it happened to me, it was while I was trying to go to sleep. I'm in my bed, then I feel a cool breeze come over me. I open my eyes and I can hear nothing. Everything begins to shake, and I cannot move. The walls started moving farther away but the ceiling starts to get closer and closer. I tell myself that if I can move my leg it will end. It takes up so much of my energy to move my leg, but eventually my leg breaks out of the paralysis, but the rest of my body is still trapped, so I force my arm to move. I start to become short of breath as I am fighting my body to move. I eventually break out of it and I am sweating.
I had that going on for about a week and a half solid (every night)
eventually it went away and I had no problems sleeping.
After a while, it happened again, but this one got weird. I woke up, and I couldn't move my body, but I could move my head and look around. My legs and my arms feel as though they were shackled and pulled apart. I tried telling myself to move my leg, and it worked, but I could not move my leg very far. I tried telling myself to move my arm, and it worked as well, but again not very far. So I'm on my bed, and it feels as though I'm strapped to the bed. I try moving all of my limbs at the same time, and then I hear a girl laughing. I begin to struggle even more. I then feel a heavy weight on my chest and I cannot breathe. I look to see what is on my and I see this girl. She had white skin, long straight black hair, dark eyes and the scariest laugh I have ever heard. She had her knees on my chest and she was holding my arms down by my elbows. She just laughed and did a backflip. She was gone and I could move again.
That was about a year ago.
Recently (within the past month), it happened again.
I was sleeping (obviously), and I was having a dream. In my dream I was on a lawn chair just relaxing. Then out of no where a bunch of dogs just jump on me. I couldn't move. I tried to fight the dogs off but I couldn't do anything. So I woke up, but I couldn't move still. Eventually I broke out of it again. I broke out of it, but I found myself in another dream. In this dream, I was trying to go to sleep. but then I heard a girl's voice calling to me. I get up to see who it is, and I see t girl, about 8-9 years old I'm only judging by height. She was wearing a yellow tea-party type dress and a white sun hat. with a red ribbon. She called my name again, and I look at her more closely. I then notice a dark shadowy figure behind the girl. Too tall to be human. it just had these blue eyes. The shadow put it's hand on the girl's shoulder, and the girls began to laugh. The both disappeared and then I woke up, and I couldn't move again. I tell myself, "These dogs were not real." I got up and it was 3 in the night. I lay back down close my eyes. My hand is off to the side of the bed, and I feel something hot and wet touch it. I tell myself, "It's only the dog," but right after I say that, I remember, "I don't have a dog" i get up and I see a shadow run past a corner in the room. The back of my hand was wet.
I never considered that it was caused by stress or bad sleeping patterns. They were just random. Or ghosts. Or even Synapses that are misfiring (or not firing). Whatever the cause, it is exciting.