The ”shadow form” is just a visual artifact of the technology they use to convert visible-spectrum light into IR/UV. It’s effective, but not perfect. The photoreceptors (rod cells) in the corners of your visual field can sometimes pick up on movement and photoreact with those frequencies.

During my sleep paralysis I would see the tall shadows moving around my bed and hear the sound of my desk fan get muffled as they’d pass in front of it.

/end schizo-post