You use the eye masks and ear plugs that they give you. And you have to have a good way of blocking out what is going on and putting yourself to sleep. I've had lots of practice at that, and not just on airplanes, which is why I can sleep on airplanes unless I'm in pain that keeps me from concentrating.
Opinions on extra long flights?
12 hours in economy class is my personal pain threshold. If the circumstances allow for longer periods of sleep, it's much better, but I can never be sure if I manage to fall asleep. For this reason, I flew from Europe to Singapore via Bangkok and rested there a few days. I also flew to the Philippines resting in Seoul.
In business class, I could survive longer flights. Perhaps, in the future 
I would prefer to direct flights. Avoid miss the second flight due to delay of first flight. And also less chance of losing the luggages.
Quoting Jessiemeow
I would prefer to direct flights. Avoid miss the second flight due to delay of first flight. And also less chance of losing the luggages.
We have long direct flights but do prefer them mostly to avoid losing luggage as mentioned above. We've had our luggage disappear for as long as three days on connecting flights although one airline managed to lose(?) our luggage on a direct flight to Paris. Obviously, they never loaded it in the first place but they did get it to us the next day. It's my favorite airline so I won't name them, but we don't count on luggage arriving with us no matter whether or not the flight is direct. Another airline lost my husband's luggage on a direct flight and he later found it in their storage room a day later so it did arrive since there were no other incoming flights during that time, just didn't make it to the luggage carousel. Many strange things happen. On our last trip, my luggage fell off the carousel before it reached the public retrieval area but a kind Customer Service lady went backstage and found it for me. That was a direct flight and we were leaving the area so really needed to get the suitcase.
The adventures are part of what makes a trip interesting . . . 