There are different types of tired. There is mentally tired and there is physically exhausted. For true physical exhaustion, there is not much else that we can do except rest. All people require sleep. It is as important as eating and drinking. How much rest is required is debatable.
It is rumored that some high achievers like Michelangelo and Thomas Edison never slept more than 4 hours. Maybe employing some sort of polyphasic sleep pattern. I used this method in college to have more study time. I find that I can sleep maybe 2 hours less in the evening if I take a 30 minute nap sometime during the day.
Here we are not talking about physical exhaustion, instead about mental exhaustion. When we just no longer have the motivation or desire to continue with our tasks at hand. We know that we are mentally exhausted if a different activity instantly energizes us. An example might be that if you are resisting doing something that will help you achieve one of your goals, and then a friend calls up and invites you to go play. If it is a person that we have romantic interest in asking us for a date, then the level energy can instantly change.
So, how can we develop this level of enthusiasm for the things we should do instead of just having it for the things we like to do? Just asking the question, leads to a possible idea. How can we learn to like doing the things we should do?
Having clear, believable, passionate goals will make working towards them something that we like to do. We generally lose interest in our actions when one of these three things are missing. If your goal is "How can I be a better person?" This goal is unclear. If your goal is "I will make US$50,000,000 by the end of this month." Unless you already make this type of money, this goal is not believable.
Check that you have clear, believable goals and that you are passionate about them. Now even with this, there will still be times when you are mentally exhausted or can't seem to take action towards your goal. The first advice is do something anyway. Don't let your feelings rule your life. Put your mind in control. Force yourself to take any small step, no matter how small.
Even with this advice to take action when you don't want to, you have the option to rest. Just like physical rest, we also need mental rest. It is OK to take a break. You can even decide to have a mental break every day. You can take 1 or 2 hours everyday, if you so choose, to do nothing towards your goals. You can watch TV, read comics, play games, surf the web, or do anything that many people might consider a waste of time. I consider this letting yourself rest mentally.
As we develop new habits, the amount of rest we want and require may decrease, so you can adjust this amount of rest time as it suits you. The point is that it is perfectly OK, to take a break, just don't make it what you do most of the time. Most of your time should be taking actions toward achieving your goals. If you are not yet spending most of your time trying to achieve your goals then review your goals and make them clear, believable and something that you are passionate about.