That sounds exactly like something that happened to me when I was started lucid dreaming for the first time.
I think it was my second lucid experience. So far, all of the most vivid lucid dreams I've had have been an hour or so before I wake up usually (I always wake up an hour or two before I should, but I can go right back to sleep). One of my first times trying, I realized I was dreaming because I saw Sonic the Hedgehog. I saw myself in third person though, and when I tried moving, I felt myself moving my legs in real life.
That wasn't the only time though, one time I span in the dream, and I actually rolled over and almost fell off my bed.
My theory is that you don't need to sleep that much anymore; your body has had its rest, so you wake up. Two days ago, I realized I was dreaming, but I couldn't do anything, and when I woke up, I couldn't go back to sleep.
When this happened to you, did you go back to sleep or get up?
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