Just a quick data point … I had planned a 24-hour fast (not a big deal - skip meals between the last dinner and the next). In a moment of weakness last night I downed a bit of chips. Processed carbs - not the best choice. Not the worst, but … ok, fine - probably the worst.
At any rate, at about the 18-hour mark, though I had not yet felt hunger or anything else, my eyesight started going wonky. I begun seeing little sparkling/twinkling lights; the longer time wore on, the more lights were present - and the less I could see. So I cut my fast short and cooked up a mid-afternoon breakfast with some awesome brown eggs supplied by my neighbor. A bit later I felt just fine - and my eyesight was normal again.
I believe from past experiences that the eyesight thing is somehow related to low blood sugar. This is something I’ve dealt with since I was much younger. I first remember the experience back in 1976. The reason I remember the year: that is when I went to a Cincinnati Reds game and got to see Johnny Bench in person.