[Lostie!]

Sleep paralysis is a phenomenon in which a person either during falling asleep (Hypnagogia) or awakening (Hypnopompia), temporarily experiences an inability to move, speak, or react. It is a transitional state between wakefulness and sleep, characterized by muscle atonia (muscle weakness). It is often accompanied by terrifying hallucinations (such as an intruder in the room) to which one is unable to react due to paralysis, and physical experiences (such as strong current running through the upper body).

It's happened to me a few times, it's really weird. Thankfully I never had hallucinations of an intruder, but I do recall thinking a member of my family was trying to wake me during my experiences of it (and when it subsided nobody was there).