Several times I've been drifting off to sleep, when I find myself thinking about something that's happened in a past dream, and it kind of startles my mind and wakes me up, at which point I realize that what I was thinking about was a part of my separate memory space for dreams.
It's interesting because I'm not even dreaming yet. Stage 1 sleep is so light that, if you wake people from it, they'll often insist that they weren't actually asleep. The mind is still thinking at that point and they'll take that as an indication that they were awake. It's considered to be sleep because people do lose awareness of their surroundings, and that's the only clue I have that I am actually asleep when it happens.
Just now I was falling asleep and thinking of a place I'd rode my bicycle where the road follows the shape of a square wave pulse, making a left turn, two right turns, then another left. I think there's like an old falling-apart barn in the path of the road, though it goes significantly further than necessary to avoid it, like the pulse is 1 to 2 city blocks wide, and 2 to 4 blocks tall. Anyway, while thinking about this, it suddenly occurred to me that, it wasn't a real place, it was a dream place. So I woke up.
I suspect my interest in this stuff disturbs my sleep a lot. I'm pretty sure that if I didn't find it so interesting that I feel I must remember it, I could have remained asleep and simply forgotten the whole experience by the time I woke up.
Today was the first time I'd had any caffeine in about a month. I wonder if that had anything to do with it.