Where Did That Come From?

I'm lying here in bed, trying to go to sleep. I was sort of drifting off, when suddenly a mental picture flashed in my mind.

I was on a landing at the top of a flight of stairs at the back of a house. I was inside, on the second floor, but the stairs extended outside. They were wooden, and painted a light green. They had no hand rail. A tall tree stood just beyond the stairs. It was tall enough that the top of it as at my eye level.

The sense of vertigo I got looking down the stairs was so strong that it completely reawakened me.

I have no idea where this mental image came from. It doesn't resemble anything I recall ever seeing.