I’ve smoked meth on and off for a couple of years. I normally stay up for three nights and I’ve never even seen “shadow people”.
I don’t actually believe in good versus evil.. I believe in light and dark, and that one can’t exist without the other. Like the Yin/Yang symbol. So maybe that has something to do with my lack of meth boogymen
I'd hazard a guess and say a combination of lack of sleep, heavy dosing of meth and the affect that has on your peripheral vision would contribute to "shadow people".
Bob are not people but dark irregular patches that dart in and out of your field of view are quite common and the mind being affected by drugs interprets this phenomenon as people that play on paranoia.
Don't forget that it's paranoia others know what you're doing as you're on the shards makes you quickly turn your head to see who is there hence the movement of the shadows.
Just a guess but the affect on the rods and cones of your eyes (control contrast and shade) woild be the thing.
Most things have a realistic explanation and reaction to them causes the shadow people associated with meth.