It's a natural chemical in the brain that releases when, usually, the body is unconcious, and during REM PERIODS so you cannot ACT out your dreams on a large scale. Usually it starts after you fall asleep and wears off before you wake up...but in some cases it can start early...like when keeping your brain concious while going unconcious...while dreaming...a WILD in other words...
It's really natural, and isn't supposed to be a scarry thing, but can be interpreted as one.