One thing I learned today is that will power is not an unlimited resource. It is definitely limited and more you use it less you have for rest of the day.
The most simpler method to explain this is to think of willpower as point. In the beginning of the day you have 100 will power points and then as you progress through your day slowly deplete it and by the evening, if you had a hard day, it is going to be 0.
This is where most of the time all the diets gets blown of and we crave for some sugary foods.
The main reason why you start craving is because sugar can increase the glucose lever like of our body which in turn gives an energy boost to our body.
Sleep is another action that can restore our willpower.
The best way to utilize our limited willpower is to use our environment to conserve our limited willpower. If we use our will power to change our environment, our new environment can help us to conserve our will power.
TL;DR
Will power is a limited resource, we careful how use spend it through out your day.