Take this time of boredom and unproductiveness to honor your mind's stillness. Do not look at it as lack of energy or mental block, but as an opportunity to reflect. Write down your thoughts (even if they seem idle and all you're writing about is the unproductiveness). It's amazing how much creativity emerges out of boredom.
I try taking a little jogging to the park question of getting the blood flowing correctly. Then, I stop for a small natural sugary snack. That helps me have a small boost of energy, however non-natural refined sugars are to be avoided.