I understand you. Sometimes there are days when your brain is "charged" and it generates interesting ideas. On such days you can create many creative ideas. On such days I write a lot and feel that this energy will not disappear. But over time, energy is depleted. Intellectual work is a very energy-intensive process. That is, when we work with "brains", it takes a lot of energy and energy. Therefore, from time to time you need to rest. If you do not rest then you will feel constantly tired and lack of energy.
In my life now there are certain difficulties. Therefore, all my energy is spent on solving these problems.