I have that issue fairly often. I think it is called sleep paralysis and, as FHC mentioned, it is usually due to the brain sort of immobilizing you so that you do not hurt yourself while you sleep. Umm...it's like...the transistions bettween the different stages of sleep...are lacking in fluidity. So instead of waking up normally, a person may wake up....mentally, I guess, first, before the brain stops whatever chemical (or whatever) that keeps you still. Umm...it usually occurs as you are falling asleep or as you are waking up. When it happens to me I am usually trying to sleep. There were a few occasions where it happened when I was waking and that scared me a lot more than when I am falling asleep because on each of those occasions I was unable to breathe. The usually scenerio involves me drifting off...and then I kind of "snap out of it" and realise that I can't move and I'll focus on one body pary (a finger for instance) and try to move it. This is very difficult, but usually works. Once I manage to move the finger it's like...suddenly I can move again. I am usually reluctant to go back to sleep at that point because I have had nights where this occured again and again. There was on occasion where I "fell asleep" with my eyes open and stared at the clock for nearly two hours....not pleasent. On other occasions I have spoken with people that happened to be in my room at the time (my grandmother, a friend sleeping over, etc) and it takes a very large amout of effort to do so. I, like FHC's friend, cut right to the chase and ask for someone to wake me up or shake me. Honestly the whole thing is more annoying than anything...so...I hope that I have been helpful.
"What happens with hypnagogic sleep paralysis?
As you fall asleep, your body slowly relaxes. Usually you become less aware, so you do not notice the change. However, if you remain or become aware while falling asleep, you may notice that you cannot move or speak.
What happens with hypnopompic sleep paralysis?
During sleep, your body alternates between REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. One cycle of REM and NREM sleep lasts about 90 minutes. NREM sleep occurs first and takes up to 75% of your overall sleep time. During NREM sleep, your body relaxes and restores itself. At the end of NREM, your sleep shifts to REM. Your eyes move quickly and dreams occur, but the rest of your body remains very relaxed. Your muscles are "turned off" during REM sleep. If you become aware before the REM cycle has finished, you may notice that you cannot move or speak."
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I think I know. I don't think I know. I don't think I think I know. I don't think I think.