First of all, I started my sketch by painting the planes of the face, which was a first for me. It's a great way to block in colors before rendering all the details.
Usually I don't give too much thought to the lighting in my pieces, so this exercise was very helpful. I didn't want to overdo it so i stopped after around 2h30 of rendering -- I'll keep practicing and i'll be adding more paintings when I have the time :) I have so much ideas with interesting lighting and shadow scenarios!
Here's another one, with a more natural light; i tried a different technique for the lighting (in the previous portrait it was mostly a blending layer on top of the painting) and i mostly used one soft round brush.