There are certainly states where the psychlogical capability to do things is decreased. If we look at a clinical case of willpower depletion we are looking at depression or other depilitating condition. People at this stage often explain their state that they have only a limited amount of units of action per day after which they cannot psychologically function, and must recharge. This is not normal state by any means, but the implications, IMO, are clear - there is a spectrum of willpower-or-similar-mental-resource available to people. At the other end are the clinically depressed with very little - the question then remains, is, how far this resource stretches on a healthy person and what are the individual differences.