Can an employer demand that I work at home without pay?

I am a physician working for a big healthcare company. I am paid hourly. During the course of my work I see patients in the clinic and have to chart patient encounters electronically. Clinics are very busy. It happens very often that at the end of my day I didn't have time to chart all the encounters. My employer requires that I complete charting at home without paying me for this work. It happens very often to all physicians in this company. I have several emails from my supervisor addressed to all physicians employed by this company demanding that we complete our charts at home. Is it legal? Thanks.