Dianne Tice explains it:
Currently, there are a few things that have been shown to improve willpower once it gets depleted. First of all is rest or sleep. Rested, well-slept people have more willpower than tired people. In fact, people’s willpower is often highest in the morning, and lowest late at night. Very few people break their diets first thing in the morning, or go on an impulsive crime spree right after waking up. Instead, these self-control failures are more likely to occur at the end of the day, leading many researchers to conclude that sleep and rest can replenish our willpower.