This used to happen to me quite a bit

The victims awake to find that they cannot move, even though they can see, hear, feel and smell. There is sometimes the feeling of a great weight on the chest and the sense that there is a sinister or evil presence in the room. And like the above reader, they are often quite frightened about what is happening to them.

It still happend to me  on planes, but without the evil presence. I "wake" when the Flgiht Attendant asks me if I want anything to drink. I am unable to raise my head up for more than a second before it crashes down. I am also unable to swallow or speak, due to what I percieve to be dry throat.    It passes in a minute or two.