well as mentioned a lot, instrumental music is what works the best
and, in my mind, some easy-to-ignore and minimalistic lyrics to occasionally stimulate your brain a bit more
another rather obvious thing is the volume.. turning it down just right works wonders
anyway, when it comes to artists this is what I personally recommend =)
Carbon Based Lifeforms
ambient downbeats at their best (to me at least)
Central Plain from their album Hydroponic Garden is a good example
Bob Holroyd
he simply makes beautiful music..
do take a listen to Hollow Man (which is both a track and the name of a truly great album)
good luck with the studying fellow students