[tungstenfurnace]

My speculation is that 'willpower' or 'vitality' relates directly to the available reserves of neurotransmitter(s) in your nervous system.

Notes:

If you try to do something bad (that part of you knows is wrong), this will set up repetitious, conflicting thoughts which act to deplete willpower.

Willpower is replenished by sleep.

Activities seemingly opposite to those activities which require willpower are known as pleasures. However, my guess is that these also drain neurotransmitters, rather like revving an engine in neutral.

Therefore the way to maximise useful activity is to:

(a) do something that is genuinely in accordance with your highest wishes and values (b) decide that it will be your sole source of pleasure and entertainment