I always believed there was a correlation between decision making and the time of day? I would have never thought it would correlate to willpower.
High intensive events that take up willpower: early morning gym, sorting out bills, morning meetings, planning a new route to go to work etc. all take a great load of the finite mental resource we call willpower.
The more complex and arduous the task the more it zaps from your system. Simple. For planning purposes this is monumental. Reason being if tomorrow you have a big meeting in the morning? Doing a 10k run probably wouldn’t do you any favours (more diminishing returns) more so than a moderate workout.
Plan ahead. Do great. Be strong.