I'm not sure when this started, but probably very early and before more fleshed out dreams started to stay with me. It occurs most often while I am falling asleep, rather than in deep sleep. It happened the most when I was 8 or 9.
Early version: Sepia tones. Inside a vertical rectangle little particles drift about lazily, with a lot of space between them. They might bump, but are not crowded. All of a sudden it gets busy and crowded with millions of little particles crowding up the rectangle.
This would make me wake up suddenly, filled with terror. It often took a long time for me to be willing to close my eyes again.
Later versions: Sometimes incorporated into fuller dreams, once set on an airplane, where people were calm and in their seats, then cluttering the aisles and moving frantically in turbulence. When in fuller dreams the cycle can often repeat multiple times without waking me up. It is also far less scary, though still unsettling.
Variations: When I was 7-10 years old, while falling asleep, I would have half-dreams where my bed would float up the wall till it started to turn to float across the ceiling. At this point, gravity would take hold and I would tumble out of the floating bed into my awake bed. I would wake up with a start, but no terror.
From 9-12 or so, I was able to control the floating sensation sometimes, so that my bed was a teeter-totter of sorts, though this was in a mostly awake state.