Sleep paralysis?

On a few occasions, during the night, I will wake up. But only for a few seconds. And in these few seconds, I am unable to move or speak. During these "events", sometimes my dreams still linger into my semi-conscious state, which makes this event extremely terrifying. IMO, this feeling, I imagine, is the closest thing to death. When the few seconds are up, I can also consciously observe a rapid descent back to deep sleep.

I don't do drugs. And I'm pretty sure I'm not crazy, although one can never be too certain of that. ;-)

Any ideas of what this could be? Is it a medical condition? Anyone else experience this? Thanks!