It happened to me when I got unemployed around 2011~2012. So, I was sleeping so late, like, 7am; staying in the computer all night long.
That made me get this "sleep paralysis".
The sleep paralysis is you suddenly wake up in the night, but you are paralyzed. You cannot move. Then, your body start to shake because you're trying to move but everything is paralyzed. And interesting enough, you start to feel that some bad spirits are close to you.