Do it anyways but take it easy! For most people it’s very hard to start exercising especially if you have been sedentary for a while. It’s hard to get that momentum going and if you suddenly stop, then it’s even harder to get it going again. There will be days where you’re tired, sluggish and just don’t feel like exercising but you should do it anyways. You don’t have to do your normal routine if you’re tired, just something light and easy so you stay in the habit of going. You can go for a walk or do some light stretching, the most important part is that you continue the routine and habit. Once you feel better you can resume your normal routine. So, the next time you are trying to talk yourself out of a workout, don’t do it. Keep the momentum going and workout anyways. Unless of course you are really sick, then it’s best for you to stay home and rest.