I have never taken hallucinogens, but I suspect I have undiagnosed narcolepsy, which causes me to occasionally experience normally unconscious sleep states in more conscious way: lucid dreams, sleep paralysis, and (the weirdest of all) being conscious of falling asleep.

My experience with the latter sounds a lot like what some people are reporting in this thread. When I am in this state where I am falling asleep, but still fully conscious, I start thinking complete nonsense — sort of like wooliness is profoundly connected the concept of leftness or like ovals are one of the primary colors (but honestly much less coherent than that).

My suspicion is that it's caused by various neurons firing stochastically as part of the process of falling asleep, and it gives me the sensation that these unrelated concepts are connected to each other. I think we are supposed to be unconscious when this happens so we don't inadvertently take meaning from what is essentially random noise.