So Nat's sig shows a hag at the base of some chick's bed. I doubt Nat knows what this is a reference to, but it's a very accurate visual representation of something called Sleep Paralysis. I've had it plenty of times in my life.

Nat's sig (it changes, so keep refreshing until you see the witch eating the girl. IF you can't find it, blame fucking Nat for being so goddamn ADHD that he can't keep 1 sig.)
[img]http://sig.grumpybumpers.com/host/natsumehack.gif[/img]

Basically, there's a neurotransmitter called Acetylcholine that gets released when you sleep and it paralyzes you like a cripple so you don't act out your dreams during the night. Except for some people like me, you can wake up during the night, but the acetylcholine is still in effect, thus rendering you awake, but paralyzed. One of the most frightening experiences ever. 

The reason Nat's sig reminded me of this, is MOST people who get SP report an old "hag" or witch-looking woman at the base of their bed, staring at them. Sometimes she says something, sometimes she just stays there and looks at you. Often times, her body will be comprised of just a large head, with raggedy, flowing hair that obscures her face (think the Grudge, but in Big Head Mode.) 
[img=http://i.imgur.com/LpFpKyi.jpg]
[img=http://thehairpin.com/wp-content/uploads/2013/10/maxresdefault1.jpg]