[lesostep]

As an artist, I was taught that when dark area meets a lighter one, I should add a slight gradient, deepening the dark near light.

Is it realistic?

No. But it looks more realistic. Because as our eyes track across real room, they adjust to the brightness.

So the darker region would look much darker near a light region – not on camera, but to human eye. And we replicate that effect in drawing, to trick the nerve behind the eye into believing that our pupil adjusted.

That's it, I think. We are not cameras, and our realism doesn't look like a photo.