It's more than overtime, see: "since half of my income from the University comes from clinical revenues generated from seeing my patients, supervising resident

It's more than overtime, see: "since half of my income from the University comes from clinical revenues generated from seeing my patients, supervising resident clinics, and engaging in weekend and holiday on-call duties. So while on leave my salary is significantly cut. Furthermore, my contract stipulates that I am not able to conduct any patient care outside the University..."