The lack of willpower is a heuristic which doesn’t require the brain to explicitly track & prioritize & schedule all possible tasks, by forcing it to regularly halt tasks—“like a timer that says, ‘Okay you’re done now.’”
If one could override fatigue at will, the consequences can be bad. Users of dopaminergic drugs like amphetamines often note issues with channeling the reduced fatigue into useful tasks rather than alphabetizing one’s bookcase.
In more extreme cases, if one could ignore fatigue entirely, then analogous to lack of pain, the consequences could be severe or fatal: ultra-endurance cyclist Jure Robič would cycle for thousands of kilometers, ignoring such problems as elaborate hallucinations, and was eventually killed while cycling.
The ‘timer’ is implemented, among other things, as a gradual buildup of adenosine, which creates sleep homeostatic drive pressure and possibly physical fatigue during exercise (Noakes 2012, Martin et al 2018), leading to a gradually increasing subjectively perceived ‘cost’ of continuing with a task/staying awake/continuing athletic activities, which resets when one stops/sleeps/rests. (Glucose might work by gradually dropping over perceived time without rewards.)
Since the human mind is too limited in its planning and monitoring ability, it cannot be allowed to ‘turn off’ opportunity cost warnings and engage in hyperfocus on potentially useless things at the neglect of all other things; procrastination here represents a psychic version of pain.
Source: https://gwern.net/Backstop