When ever you are going to travel for more than an hour you must consider what exercise that you will need.

I am regularly flying 12-15 hour trips and ensure that I walk around the cabin a couple of times (avoiding meal service time).

While I am talking about pilots, the pilot numbers on flights differ depending on the length of the flight. A two person/pilot aircraft can be flown up to about nine hours before another pilot has to be carried for in-flight relief/rest. Most airlines take another pilot for flights of 12 hours or longer. The reason being that there are always two pilots on the flight deck. On long flights extra pilots are carried to enable other pilots to take a rest. Would you be happy to fly on a 14 hour flight with just two pilots? Imagine how tired you would be at the end of a 14 hour flight and have to land your plane. Not many people go to work for 12-15 hours at one stretch and the most important part of their work period is at the end, the last couple of minutes. So pilots need to be alert and, to achieve this, rest breaks are necessary.

So passengers need to look after themselves as well. I said at the beginning you don't sit still at home for 12-15 hours, why would you expect to do so on a long flight?