Hat man phenomenon. It's a very common sleep deprivation/sleep paralysis hallucination. It's actually pretty interesting, since it's only been common for a few decades, likely due to movies involving tall scary men with hats.
Madness string 111266
It takes a very special and very strong-minded kind of atheist to jump up and down with their hand clasped under their other armpit and shout "Oh, random fluctuations-in-the-space-time-continuum!" or "Aaargh, primitive-and-outmoded-concept on a crutch!"