When to Give in to Exhaustion

How much should I push myself when tired? I work a full time job (at a desk) and my husband works a full time manual labor job with lots of overtime. In the evening I feel I have just enough time to eat dinner, rest, and help my son with his homework before I'm too tired to do anything else, but I feel in the process of getting enough rest my house isn't getting the attention it needs as far as me cooking and cleaning. Should I push myself harder? I don't want to be a complete slob by the time the baby arrives