A few years ago I started to feel really run down to where I would instantly want to get my pajamas on and rest some place warm after getting home, even if it was still light out. It turns out I was anemic and iron deficient. The doctor put me on some iron pills, which rectified the problem. You shouldn't assume this is the cause of your fatigue though, because it might not be, and it can be dangerous to take iron pills if you are not iron deficient. As Jan suggested, I would have some blood work done. Also have them check your thyroid.