There are regulations that apply to long haul flying. Those are done by widebodies. Any “narrowbody” with a 15 hour endurance would be a biz jet and therefore, not subject to these regulations. It basically goes without saying that any regulation that applies to long haul is going to be widebody. As far as to what is fatiguing, I don’t know, 18 hour days, landing and taking off at night, sleeping during the day, trying to rest in a crew rest facility, a hotel used by Arabs as a brothel, circadian rhythms, bad food, backside of the clock. That kind of stuff wears me out. I kind of a wimp though I guess. I’m closer to 40 than 30 these days too.