Not all long journeys are equal. The top 3 hell routes I described earlier are hard for me for reasons that are not necessarily tied to the duration of the flights. Here are some factors that influence how hot it gets in hell:
1. Timing: time of departure and time of arrival matter. A lot. If the flight arrives in the afternoon or evening, I find it much easier to adjust to a new time zone and skip the curse of globetrotters, jet lag. The worst of the worst for me are flights between the US East Coast and Western Europe. They typically leave at night, which means I'm already tired from working a regular day. Dinner is served an hour or so into the flight, and breakfast an hour before landing. That leaves at best 4 hours to sleep. Landing at Charles De Gaulle at 7am with barely any rest and an entire day to survive before going to bed is just misery.
2. Flight sequence: a short flight followed by a long flight is generally better for me. Nothing worse than arriving exhausted after a 14-hour flight and still having another 7-hour flight to 'look forward to'. That's what makes IAD-SIN one of the hottest hells of my travel life.
3. Time zone difference: I adjust better with a clean flip of night/day, like going from the US to Asia (typically a 12 to 14-hour difference). Somehow that works better than even a smaller but weirder time difference, like the 5 hours between DC and London. True hell is India or the Middle East with 8-10 hour differences.
4. Layovers and airports: I like my layovers short and sweet, but that can be risky at airports like Heathrow, where I wouldn't dream of booking connecting flights with less than 90 minutes between them. Singapore's Changi, on the other hand, offers so many ways to pass the time that it almost begs for a long layover. If there's too much time between flights (like a 6-hour layover I had at Heathrow connecting to Johannesburg on BA), transit hotels are life savers. The hell potential is high at airports where there is nothing to do, the layover needs to be long, and there is a lengthy flight coming after an already tiring journey. I'm looking at you, Manila.