Here's what I see when I am about to fall asleep:
A blob of faint blue, sort of a violet blue, congeals in my visual field. It shrinks, and as it shrinks, the color intensifies. It shrinks down to point and disappears. About that time, another large, faint blue mass is forming, and then it shrinks too. Each cycle takes a few seconds; the entire effect is one of travelling slowly backwards through a diaphanous blue tunnel.
I'm not sure when I first became aware of this, but anytime I pay attention, it's there. At the same time this is happening, I might have images in my head of things I've seen that day, stuff that's happened, etc., but the blue blobs are different. They are "really there" in my visual field, and, unlike the random things that pop into my brain when I am falling asleep, I can observe them with intent.
A similar thing happens to my Mom, but hers start as a point and get bigger and fainter, eventually dissolving. So I guess she's going forward through her tunnel. When we were having the conversation in which I learned this, my aunt and uncle (people I have very little in common with) were present, and they they thought we were both whacked.