Willpower fatigue, also known as self-control failure, has three major causes, according to Florida State University Professor of psychology Roy Baumeister. First, conflicting goals and standards undermine control, such as when the goal of feeling better immediately conflicts with the goal of eating less in order to feel better in the long run. Second, failure to keep track of (monitor) one’s own behavior renders control difficult. Third, self-control depends on a resource that operates like strength or energy, and depletion of this resource makes self-control less effective. Having to make a lot of decisions (“decision fatigue”), even minor ones, has been shown to deplete self-control.
See: Baumeister RF: Yielding to Temptation: Self-Control Failure, Impulsive Purchasing, and Consumer Behavior. Journal of Consumer Research, volume 28 (2002), pages 670–676;
also, Vohs KD, et al: Decision Fatigue Exhausts Self-Regulatory Resources. Feb 15, 2006 http://www.gsb.stanford.edu/facseminars/events/marketing/pdfs%202006/2006_02-15_Vohs.pdf.
[The above information is excerpted from Weight Management for Your Life]