Does sleeping in an airtight room (window close) affect sleep quality and muscle mass?

Not: A room with the window closed isn't really airtight. If it were, you'd use up all the oxygen and die. A truly hermetically sealed room is hard to find unless you work in a high-tech facility that has them, and knocking one together in your garage tends to be above the average hobbyist's skill set. So I wouldn't worry about your room being airtight with the window closed because it really isn't.

Probably not: A stuffy room may affect your sleep quality but not likely your muscle mass in any significant way. If you are asking because you have experienced a loss of muscle mass, weakness, fatigue or any other specific symptoms, then you should consult your Dr.