In late 2003 I started to see illuminated geometrical shapes, sometimes flashing, as I was drifting off into sleep.
This was often preceded by a sensation of falling into a pit or vortex.
During the same period, I often saw momentary images, often bizarre faces, when in a half-asleep state in the morning.
I found these phenomena pretty disturbing, and worried about the possibility of a brain tumour or similar.
Several months later I researched the matter, and found the former to be "hypnagogic hallucinations" and the latter "hypnopompic hallucinations".
From what I recall, these were said to be surprisingly common, and not indicative of any brain malfunction or abnormality.
I had these experiences for a period of about two months, and have not been troubled by them since.