The thing to remember is that the travelling itself is exuausting, even with lounges and F class lie flats. I don't care how comfy you are, LAX to HKG or BKK for example would destroy even the most seasoned flier if they got no sleep. And landing is not the end of it. You still have to negotiate through customs, find your ride, and find your destination. In a state of exhaustion, you make mistakes. You get confused. Your emotions get completely out of whack. You can even get physically ill..my wife starts throwing up after 24 straight hours of travel. Exhaustion is not too different from being intoxicated, and can become debilitating. So best is to eat as much as you can, because caloric energy can stave off the worst aspects of exhaustion, sleep as much as you can to clear the acetylcholine out of your brain (it's what keeps you awake but builds up over a day and becomes harmful unless cleared by sleep), and stay hyrated.

I sometimes feel that the steps taken to avoid jet lag run counter to managing exhaustion. Skipping a meal, eating at odd times, and avoiding sleep..if you are going to do this, do it DAYS before your trip, not simply in the last 24 hours before you arrive. The gains seem minimal doing it in transit, vs taking care of your own condition enroute.