The common idea is that willpower is limited. After 4 hours studying mathematics, you  are likely to feel like entering a coma-like state on your couch and binge on Netflix. 

However, Carol Dweck and team believe otherwise:

It appears ego depletion may be just another example of the way belief drives behavior. Thinking we’re spent makes us feel worse, while rewarding ourselves with an indulgence makes us feel better. It’s not the sugar in the lemonade that produces the sustained mental stamina, but rather the placebo effect at work.

Maybe willpower is an emotion?

Michael Inzlicht, a professor of psychology at the University of Toronto and the principal investigator at the Toronto Laboratory for Social Neuroscience, believes willpower is not a finite resource but instead acts like an emotion. Just as we don’t 'run out' of joy or anger, willpower ebbs and flows based on what’s happening to us and how we feel. Viewing willpower through this lens has profound implications.