I don't find it onerous in a overall way. I just sleep when I need to sleep irrespective of time of day.
To some extent your body clock becomes 'permanently' adjusted to the average time zone you spend most time in...in my case it seems to live somewhere over India enroute to the ME
Certainly going east can be a pain. When my home time zone is SEA and my body clock averages somewhere west of there going to AKL can mean seriously disturbed sleep. Luckily for us our schedule doesn't have us leaving AKL very early and we don't go very far...BN...and then get off for up to several days. So despite laying awake until 0500 LT because your body clock thinks it's 2200 the night before I do get enough sleep and feel quite ok flying across the Tasman.
Going westbound is a doddle and coming back east to your normal time zones just requires a little planning to ensure you're fit to fly.
While young blokes should be very much more tolerant they usually also feel the need to 'socialise' downroute a great deal more than those of us who have that out of our systems...thus they often suffer more. Much older pilots possibly suffer more too because of aging effects but it seems to me those of us between those extremes fair reasonably well.
I am naturally a bit of a night owl anyway and I think that helps. Downroute too you're not influenced by family to get up at a certain time and usually don't have any running around paying bills pressures...so you just sleep until you wake up. At home my child is late teens and somewhat of a night owl herself...when I am home I go to bed when I am tired and wake up at whatever time and don't care if that is lunchtime. I work other aspects of my life around that. Of course if I need to be up I get up and have a nap later on...if I don't I don't.
Our company schedule has most long haul flights departing early evening from home base and around midnight at some ports downroute. Some pilots feel the need to use chemicals to sleep but that just terrifys me...or the thought of becoming dependant does...I avoid them completely.
All in all after spending a career doing shorthall and conciously avoiding longhaul I finally found myself doing longhaul and I have to say I prefer it.
