“You’ll be lying in bed and you can’t move, you can’t talk"
My god I've experienced this quite a many times and its like some ghost or something sitting on you during sleep. After talking to many friends, doctors and folks back home it known as "Sleep paralysis". This happens especially during the REM part of the sleep (REM - Rapid Eye Movement).
Read more at Wiki. The Nightmare, by Henry Fuseli (1781) is thought to be one of the classic depictions of sleep paralysis perceived as a demonic visitation (above picture).
How to cope with it ? Well I'll say just ignore it and maintain calm. Else read this link.
Also a review from the stanford group.
I guess after the Jaw Locking post this is the most scary thing that I've experienced.
Pic Courtesy Wiki.