Well, it sort of goes against 'being in a bunker', but if I was going to work somewhere for many hours, I'd like some natural light.

Of course it's still possible to achieve that using reflective tubing or such like, though it might still undo whatever it is you seek to achieve by being underground.

If it's not possible, I'd suggest paying lots of attention to lighting. And add some real plants too - they'll generate oxygen as well as making the environment seem less bunker like.