Psychologists now think people have a willpower stat, IRL. Okay, they call it ego, but I'm going with Willpower Stat. Basically, we all have a finite daily amount of Willpower that runs down as the day goes on and we deal with small annoyances, temptations, and so on. Then it recharges overnight. The theory is, if you want to do something that is important to you, but you are struggling with it, like learning a new language (or writing a book) do it really early in the morning, make it one of the very first things you do, whilst your willpower stat is at its daily highest.