I'm really glad to see people like you, motivated to exercise despite a tiring schedule. Really, you are not alone. Many people are also tired, stressed from work and lack the zest to exercise. You just gotta find the time to do it. The benefits that follow is just worth it. For me, i usually run in the morning at about 6 or 7am before heading off to work. It would be painful at first, but soon you will see results (you'll feel and look fitter) and these results will motivate u to exercise more and more. Same applies to lifting weights. Usually i lift weights at the gym 3 times a week after work. If i'm feeling too tired, i'll take a quick high sugar drink or pre workout shake to kick start the workout.