Caravaggio was the master in using light and shadows in his portraits and paintings.
At the academy we worked several lessons on this clair-obscur theme.
First from a statue with a large spot posted on the left side:
Then a second lesson on the same subject but with different result:
And then with a model, he was put in the light and we were working in semi-dark settings.
Starting from the dark/black and then putting more whites into place...
it wasn't easy, but hey Caravaggio is a master and must have practised and been drawing a lot to come to his masterpieces. You can find more on his work here.