Have you ever woken up suddenly out of a dream, only to find that you are incapable of moving?
At this point you would probably start to panic, and the fact that you might even sense another presence in the room doesn't help either.
What is actually happening to you is that you are experiencing Sleep Paralysis.
Sleep Paralysis is a phenomenon which occurs upon either entering or awakening from sleep. The effects are complete paralysis, and the inability to speak; most who try can usually get no more out than a strained groan.
Another common and particularly interesting effect that occurs during Sleep Paralysis is that many who experience it have reported feeling a heavy weight on their chests, as well as feeling an 'evil presence'. Some have even seen visual images of dark figures.
Sleep Paralysis occurs more frequently among those who have narcolepsy. It is also easier to happen when you are very tired from staying awake for too long.
Apart from the supernatural theme and explanations for this phenomenon, there is a perfectly informative scientific explanation for why this occurs.
To start, there are 5 stages of sleep. The most relevant stage however, is Stage 5: REM Sleep.
REM stand for Rapid Eye Movement, and it is during this stage that we are in our most deepest sleep, and when we dream. Now during this stage of sleep, in order to keep us from moving around in our sleep from our dreams, our brain releases a chemical which stops our muscles from reacting to our brains signals. Therefore, every night when you sleep and you dream, you are, in fact, paralyzed.
Now the reason Sleep Paralysis occurs, is that for some reason, you can wake up during your REM stage, and usually at this point the chemical is supposed to cease from being active, but sometimes it doesn't. It is because of this that Sleep Paralysis occurs.
Note: From this we can see that Sleep Walking is the opposite of Sleep Paralysis, in the sense that Sleep Walking is when the chemical ceases to stop the muscles in our body from reacting to that of our brain, thus ending up with us walking around and acting out our dreams.
Also, if the body is more exhausted, this can make it much easier for the chemical to stop working, simply because the body is still asleep, which is why it occurs in people who have narcolepsy a lot more.