There are multiple factors which cause fatigue. One of them is poor nutrition. The obvious one is improper rest. Circadian rhythm shifts are another. Some people wait until they are draggin ass to call. I don't let myself get to that state. It's not good for my passengers or for me.
I know my body well enough, and have been around this gig long enough to know when I'll be properly rested. And I take the necessary steps, afforded by agreement, to ensure I'm well rested, well nourished and safe to fly.