How hard should I push myself?
I think a good rule of thumb is that people are strongly overly biased towards inaction and laziness. Humans evolved in a calorie-scarce environment. Conserving energy and effort was literally a matter of survival. It makes sense to select for lazy behavior, especially when rewards are intangible or not immediately guaranteed.<p>Of course in the modern world, calories are too cheap to meter. Our instincts no longer make sense. It's almost certainly the case that our laziness, driven by a vestigial impulse to conserve calories, far exceeds what's optimal or necessary.<p>The one exception to this is the tiredness of sleep deprivation. Critical biological functions occurs in deep sleep, and being exhausted due to lack of sleep cannot be made up with extra calories. The best advice is to never allow yourself to become sleep deprived. That removes any ambiguity about whether a perceived lack of energy is genuine exhaustion or can be powered through with willpower.