Personal Energy Management
I just read an interesting Dutch article on a concept somewhat related to
time management: personal energy management. Here's a brief translation...
WHAT IS PERSONAL ENERGY MANAGEMENT?
Like everyone else, you have periods in which you feel really good and that you are having a good time and that you are productive. On the other hand, you also have periods that you feel hungry, thirsty or tired, and you seem to be unable to get anything done and have no inspiration. And finally there are periods when you need to rest.
DECIDE WHEN TO WHAT
The amount of energy you have is limited. So manage your energy properly. In particular, consider at what time of the day you have the most energy. For example, it could be that in the morning you are able to concentrate better, have more inspiration and are more productive and effective.
If that is the case for you, you should move certain important tasks to the morning. You can decide to focus on the most important project or on writing an important article in the morning and spend the afternoon on routine tasks like perhaps checking and answering your emails or performing other operational stuff. That is where a connection with time management exists.
THE IMPORTANCE OF GOOD SLEEP

Personal energy management also means that you sleep well. For most people 8 hours is a minimum so that you are recharged the next day. View yourself as a big rechargeable battery… At night when you're done with your work, you have to recharge your battery, in order to be fresh the next day. Do not believe that by working 60+ hours per week, you'll get a lot more done.
Instead, consider how much you can actually do in the approximately 40 working hours (it doesn't matter how many hours you work precisely). The point is that you will be a lot more productive in the hours that you do you work if you recharge on time!
TAKE REGULAR BREAKS
After you worked for 1-2 hours, you probably depleted your energy and you become a lot less productive. You could be thirsty and/or hungry and feel a bit tired.
Then take a 15-30 minute break to take a walk in the garden or perhaps do some exercise. You have to recharge yourself completely in order to be able to continue your work for another period of 1-2 hours.
Too many people make the mistake of working for 3 or 4 or 5 hours without any interruption.
Source: Blog at winst.nl (in Dutch)