I recall learning that the Arizal would sweat profusely when learning nigleh and that this was related to removing the klipot. I cannot recall the source for this; does anyone know?
Pri Etz Chaim - Shaar Hanhaga Halimud
And my teacher (the Arizal), of blessed memory, from his tremendous quickness and depth, would study a halacha (talmud) in 6 ways in the manner of pshat (simple meaning) corresponding to the 6 days of the week and afterward he would say the halacha in the manner of Sod (secret meaning) corresponding to Shabbat...
I also saw that when he would read a halacha (talmud), among the group, he would question with great force until he would exert himself tremendously and sweat profusely. I asked him why he does this and he answered me 'the toil in halacha (talmud) is to break the klipos (evil inclination). This is the secret of the kushyos (tough questions), and for this a man must toil greatly. This is why the torah is called "tushia" - it weakens the strength of a man, and likewise it is proper for a man to arouse himself and to break his strength (in learning talmud).