Sleep Paralysis.

It happens usually when you are ALSEEP, in REM sleep cycles.
It is a natural chemical released in the brain so we DON'T act out our dreams.

Now...sometimes the chemical gets released before REM SLEEP, or can endure after REM sleep, when you wake up.
When attempting WILD, this usually happens to alot of people, because they stay concious when their body, or rather brain is going subconcious.

REMEMBER: it is natural, happens to everyone, and don't be frightened, try and relax.