I am so sleep-deprived at this point that I am having hallucinations! That's right, classic visual hallucinations. Although before I had any training I didn't know that's what they were. I would have called them "swirly spots in my vision from being tired." Mine usually follow a pattern where there's what looks kind of like a ring of distorting glass in my field of vision. (It's a similar sensation to looking at something through your blind spot.) But not only is it distorting, it also has kind of vibrating black-and-white stripes running through it. It's very distracting when I'm trying to work. Inevitably the ring starts out very small and fat near the center of my field of vision, like a little solid circle, and then gradually grows larger and more attenuated. Once it's out of the center of my field of vision, I can work sort of "through" it as it grows larger and eventually fades.
Funny, because just last night I was just talking to a relative who called me in a panic because a doctor had suggested she might have hypnagogic hallucinations, and she thought that meant she was crazy (whatever that is.) But anyone can have those, if they stay up until 4am and then get up again at 6. Not that I ever did that, nor did the complex hallucination I didn't have involve a robot standing inexplicably at the foot of my bed.
(However, auditory hallucinations are much more common than visual ones.)