when it happens to me my treatment providers cal it sleep paralysis.

the first time I asked about it my treatment providers explained to me that when the brain gets ready for the body to sleep it triggers a chemical release reaction in the body. this release of chemicals makes it so that a person isnt physically acting out what ever they are dreaming, its the body's natural safety lock to prevent someone from say walking out their door and jumping off the near by bridge and accidentally killing their self when they are dreaming about jumping off the bridge, or having a dream of killing, having sex or what ever with their friends, the person wont wake up to find they actually did harm or have sex or what ever with the person they were dreaming about.

usually people are not aware when their body chemicals release and effectively render them unable to move but sometimes with some people their body goes to sleep while their minds are still conscious, or they wake up before these chemicals wear off.

either way its happened to me and can be quite scarey. what helps me is reminding myself that this is just a normal body function, lay there and breath and take time to slowly wake up. sometimes I wake up my wife and tell her its show time lol she laughs and turns on the tv, places my head phones on me and she goes back to sleep or gets up to start her day depending upon when it happens. and I wait out my body by watching a movie or the morning news.