The Science of Willpower
Willpower sounds cryptic. We’re either disciplined or we’re not. Some people seem to ‘just have it’ while others don’t. Yet, modern science has dissected the phenomenon of willpower carefully. In essence, willpower functions like a muscle and can be both trained and depleted. There are certain factors that determine how much willpower you have, factors which you can understand and leverage.
Walter Mischel’s Marshmallow Experiment
The importance of willpower is often highlighted by the Stanford Marshmallow Experiment, which I highly recommend you watch before continue reading this article. In the video, little children are confronted with a choice: They either get one marshmallow now, or two marshmallows later. Yet, the task comes with a difficulty: The children had the marshmallow placed on a plate in front of them during the entire waiting time. Only if they did not eat that marshmallow within the first ten minutes did they get the second one. 
Interestingly, this experiment was part of a long-term study. The kids that made the entire 10 minutes were not only funny to watch, they were much more accomplished in later life. That is, those that were able to delay gratification were described as much more competent individuals by their parents. Moreover, in a later follow-up study, a correlation between SAT scores (an intelligence test for college entry in the US) and delaying gratification was found. Another study found differences in the brains of those people that delayed gratification. High delayers had a more active prefrontal cortexes, which orchestrate goals, thoughts and actions, i.e. define people’s ability to plan and be disciplined. Studies even found correlations between delaying gratification and body mass index (BMI). 
The scientific workings of willpower were discovered later by Roy F. Baumeister. In one of his experiments, he led people into a room that smelled like chocolate and showcased a variety of cookies and other sweets. One group of participants was allowed to taste the chocolate, while the other group of participants was given radishes instead. Baumeister even notes in his research paper that people from the latter group starred at the displayed chocolate or even grabbed the cookies to smell them. Yet, this seemingly evil experiment didn’t end there. Baumeister had his participants solve a seemingly unrelated task afterwards: Solving a persistence-puzzle. Most interestingly, the group which had to try radishes made far less attempts in trying to solve the puzzle. 
With this experiment, Baumeister laid the foundation for over 1200 subsequent studies on will-power and discovered one of the most important phenomena around the subject: ego-depletion. He explains: Willpower behaves like a muscle. Once it’s exercised, it depletes and needs time to regenerate. The group that had to eat radishes instead of chocolate had to exercise their willpower and thus didn’t have enough of it left to try hard at the puzzle.
This finding triggered another alternative explanation of the marshmallow experiment. As you could see in the video, many of the children that made it used tricks that helped them to distract themselves. These tricks allowed them to circumvent the usage and thus the depletion of willpower. Many more studies afterwards, which focused on self-control, demonstrated this effect further, for instance by applying tricks to dieting. Tricking yourself into dieting is much more effective than doing it wilfully. The reason: ego-depletion. 
Rethink your daily structure. Are you performing tasks that deplete your willpower but could be avoided? Are you performing tasks in the evening that require willpower and should hence be done in the morning? Adjust your day so that it makes sense considering ego-depletion. A good rule of thumb is: Start with the most challenging task in the morning. And: If it’s late in the evening or you know your willpower is depleted, don’t start any challenging task at all. The effects might be much more detrimental than simply postponing it.
 Mischel, W., Ebbesen, E. B., Raskoff Zeiss, A. (1972). Cognitive and attentional mechanisms in delay of gratification. Journal of Personality and Social Psychology. 21 (2): 204–218. doi:10.1037/h0032198.
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 Schlam, T. R., Wilson, N. L., Shoda, Y., Mischel, W.; Ayduk, O. (2013). Preschoolers’ delay of gratification predicts their body mass 30 years later. The Journal of Pediatrics. 162 (1): 90–93. doi:10.1016/j.jpeds.2012.06.049. PMC 3504645. PMID 22906511.
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 Baumeister, R. F., Bratslavsky, E., Muraven, M., Tice, D. M. (1998) Ego depletion: Is the active self a limited resource? Journal of Personality and Social Psychology. 74, p. 1252–1265.
 Baumeister, R. F., & Tierney, J. (2012). Willpower: Rediscovering the greatest human strength. New York: Penguin Books.