Just in case anyone is curious about more of the physiology of sleep paralysis...
There are different stages of sleep that everyone goes through every time they sleep during which different things happen in your body. One of these stages is REM sleep (Rapid Eye Movement) during which almost all of your muscles are paralyzed except for your heart, some of the muscles that allow you to breathe (I can't remember which ones at the moment), and your eye muscles. A lot of dreaming takes place during REM sleep and people's eyes move around a lot, hence the name. Usually a person doesn't wake up while they are in REM sleep so most people never know that they were paralyzed. On the rare occasions that people do wake up directly out of REM sleep, they find themselves still paralyzed which can be quite scary to say the least. Lack of paralysis during REM sleep can actually be a really bad thing as the person usually has REM Sleep Behavior Disorder. Since the person is not paralyzed, they physically act out their dreams which can often be violent. Often, the person is dreaming that they are under attack and so they defend themselves, which can unfortunately lead to their partner to waking up in the middle of the night because they were just punched in the nose. Not such a good situation. REM sleep behavior disorder can sometimes be a precursor to developing Parkinson's disease. (Sleep is NOT actually one of my special interests: my dad is a doctor specializing is sleep, so I've grown up with words such as "REM sleep" being common everyday terms, e.g. that's the reason I know all this random stuff about sleep.)