I do know how you feel as I am doing my PhD and at the moment it is full on (up to 60 hours a week) and the on top of that I do try to exercise most days because it keeps my stress down and otherwise I get headaches. But I do feel like I am missing out on DS time as well. The thing that keeps me going is that firstly I will get a better job afterwards and secondly it is only short term (another 6-12 months) and then I can take a whole year off. So I guess I can't really give advice but just say that I know exactly how you feel.