Whenever I sleep when it’s light out, such as an afternoon nap, I have extremely vivid dreams. Ditto for if I fall asleep with a light on.
Why?
My theory is that the light is stimulating my uhh, eyes and stuff.
I don’t have any answers but I find the same thing. I have more frequent vivid dreams if I sleep in the light.
WAG: Light is distracting and more difficult to fall alseep in, so you tend to fall into a deeper sleep than you do at night. It differs from falling asleep in, say, a noisy environment because it is not as much of a distraction and may allow you to fall more firmly into REM sleep.
When you say vivid dreams… never mind. 
My WAG would be quite the opposite - I would propose that the light keeps us in the mysterious state of hypnagogia on the border between sleep and wakefulness. Both lucid dreams and hypnagogic hallucinations (believed to be the basis for alien abduction fantasies) occur in this state.
I’d never thought about this; I fell asleep with the light on last night, and while I’m not sure if my dreams were more vivid, they were certainly stranger, and I was able to remember them for longer than 30 seconds.
I can still recall parts of it now; airliners were making belly landings in a forest, where people waited to provide sleigh rides. I’d won a radio show contest, and our office administrator traded me a bottle of rye for my bottle of vodka.
No cigars though. 