I can sleep around 8 hours during these long flights. Some tips (if you want them):

- I prefer a place next to a window. I can put my head on the wall.

- I try not to drink too much. Definitely no coffee or alcohol. Not having to pee will make me sleep longer. Going to a toilet in a plane is difficult.

- Before I go to sleep I read for 1 to 2 hours. I try to avoid something that is exiting to do (playing video games, doing intense difficult programming, ...)

- I take somewhere between 0.5 and 3 mg of melatonine one hour before I want to go to sleep. This helps me getting tired in areas where you have a lot of light.

- I wear earplugs during sleeping.

- I cover myself with as many blankets as possible. It can be cold in airplanes, and getting too cold will wake me up.

Good luck!