This may be anecdotal; please feel free to vote to close.
After staying awake for an extended duration, I usually find myself having to take extra effort to focus my eyes on an object. Similarly after exerting myself to the extent of near total exhaustion I find it necessary to consciously focus to perform actions that would otherwise require little conscious effort (E.g. Writing a note). There're other similar symptoms almost all of which mandate 'conscious effort'.
What really happens when a body is tired? As a corollary, why does making conscious effort usually beat the initial error?