Time management and energy management are two totally different things.  If you can believe that time can be "managed" (many don't) it is about allocating blocks of time to tasks to maximize what gets done in a day or in a week.  However, underlying this basic time management philosophy is that all the time blocks are created equal in your ability to concentrate and focus and that you have constant levels of energy and capability for each time block.  We all know, however, that this is incorrect.  Just recall the time that you were trying to complete some complex or complicated task late into the night where the head bobs of fatigue are being fought and concentration is at it lowest.  Or think about how doing the same task in the morning when you are fresh is so much easier.  This is the essence of energy management - matching the task to the energy requirements that you can deliver.  Using your best brain power time to simply answer emails will leave you depleted when you need it most, or trying to do a high intensity workout later in the day when your are just worn out from the day will simply cause you skip more training sessions than you complete.  Being more rigorous with your scheduling and planning can ensure that you can get more done, be more productive and feel better about yourself simply by matching the task with the right energy level.

And what is a man without energy? Nothing – nothing at all. Mark Twain

The energy of the mind is the essence of life.  Aristotle

Energy and persistence conquer all things. Benjamin Franklin