the folk term
is 'Old Hag' and it can be terrifying, it is supposedly caused by a chemical your body releases while you sleep that prevents you from moving around too much and hurting yourself, if you sleep on your back it collects, and when you wake up it takes a while to dissipate, leaving you frozen temporarily.

The best way to combat it is to relax, but depending on what shit your mind throws at you this can be hard, but since you are technically asleep while being mentally awake, with training, you can turn it into a lucid dream.
(Lovefist233 Bothering Sheep since..., Thu 3 Jul 2008, 17:09, Reply)