I feel this way too sometimes. Anxiety pushes your body to the max emotionally and physically. Thus, I often feel tired on some days as my anxiousness wore me down the previous day or earlier that day.
I find that it is good to avoid caffeine if possible. It is only a temporary re-fueler, and later often causes more anxiety. If desperate, I drink a small cup of tea, as tea has been shown to increase energy without causing the jitteriness of coffee (I think black teas are esp. good for not causing jiteriness).
A diet rich in healthy fiber and carbohydrates (not sugar) seems to help regulate energy throughout the day. I think it is also important to take breaks throughout the day, to just sit and relax and recharge. Sometimes a nice hot showers helps wake me back up too, especially if I use one of those cologne soaps like old spice, etc. The smells are awakening. Plus, up beat music helps too. If I am too tired to exercise, I will listen to my MP3 player for a while, start off exercising slow, and the music usually helps get me get moving again at my normal pace.