I've always wondered why airlines don't have stacked bunks on longhaul routes, as an option, rather like the couchettes that you get on trains but closer together, for space economy, not comfort. Without analysing it, I would have thought that you would get more people lying flat onto a given space than seated, even in densely configured economy class.
There is wasted space above seated passengers' heads, and if the 'bunk' were closer to the cabin floor, you could get two or three levels of bunks
First of all , there ARE stacked bunks in the overhead areas of large transcontinental aircraft. But for crew only. see link below.
The Part of the Plane You Never Get to See: What Do Cabin Crews' Chillaxation Spots Look Like? - Core77
But these bunk areas (like the lavatories) are NOT certified for occupancy during takeoff, landing, and taxi. Crew members must be in crew jump seats during those phases of flight. Neither the crew seats nor the crew bunks are certified for passengers, only trained crew members may occupy them.