People usually find after feeling tired they experience a surge in energy. The theory is that the body responds by producing more blood cells and this results in kind of a over compensation.
You can look at it kind of like having a glass of wine. The first time you have a glass of wine you feel the effects right away. However, if you have a glass every night with dinner, the effects are greatly reduced.
In both cases the body is stressed, but after repeated exposure it tolerates it better.