It is very insightful to spend even a short amount of time hitting somewhere close to maximum productivity and effectiveness.
You can't really see or feel opportunity cost when you're kind of screwing around, half-working, not getting much, done, etc. You're not getting as much done as you could, that's true, but the suspects for what's dragging on you are not about the other productive endeavors, but about all the distractions.
As soon as you're near peak production for even a few days in a row, you start to see the interplay of how we all have a single well of time each day, 24 hours deep, and it gets drained systematically as the minutes pass.
There is much to be said about this point, about opportunity cost, about focus, about achievement, about whatever else — but we won't say that now.
Rather, let's talk about the days when the water in your well of time is hard to drink, and not so much is happening.
Bad days happen for whatever reason: maybe you're minorly ill, fatigued, ate something poor, "woke up on the wrong side of the bed," or whatever else. Everyone has at least some variance in the quality of their thought and feeling and action, and some days this will sum to not-very-good.
Having will is good and gives many blessings, but having will gives way to being stubborn very easily.
It's axiomatic that you're going to get less done on your days that go off the rails.
And that's where willful people get into problems. As it gets later in the afternoon, and then the evening, and whatever you had decided to do that day still isn't done... it can be very tempted to keep tenaciously and stubbornly grinding at it.
And sometimes that's even the correct call, if you have deadlines or there's some particular necessity to grind to keep your word, and if the production will actually work.
But here is the issue: when you run yourself into the ground on a bad day, you're draining the next day's reservoir to do so.
In effect, you're shifting time and energy from a day of unknown quality (tomorrow) to a day of known poor quality (the bad day you're having).
In general, just due to the law of averages and regression to the mean, the next day is likely better than a very bad day you're having.
Furthermore, if you're having a bad day because you're literally physically exhausted, resting might be a requirement to get back to peak production. Pushing yourself harder on this bad and staying in the fatigued and "feeling run down" state is obviously counterproductive.
Yet, it can be very hard to give up on a day when things aren't clicking. Especially if you're naturally strong-willed.
But wisdom and effectiveness are certainly more important than raw will; will is there to serve us as we go on to do things we find meaningful.
And of course, there are many caveats. Sometimes grinding through is the right call. This is tricky to decipher.
But as a general and overarching, important rule,
Never take actions on a bad day that drain the next day's reservoir of time and energy.
There is no shame at all in pragmatic rest, recovery, and adaptation to what's going on. On a single bad day in the midst of a good run, scale back your efforts to what matches what you've got, rest, recover, and wake early the next day to take the world by storm.