I have done 10 and 12 hour flights. It is unbelievably boring, especially if you are following the night round the earth over the arctic circle so there is nothing to see. You can get up and walk around, go to the loo several times. Often, they put left over snacks, juice and water out for people who are not sleeping (they put the lights down so you can pretend it is night, even though it is the middle of the afternoon in the time zone you set off). In the very big plane I went on, you can take a turn round the deck, but not up and down the stairs. On the smaller one, they put the stuff at the back so you could go to the loo, have a drink, and look out of the window.
I expect the shorter flights are worse, because you think it is not worth getting up and try and hang on to use facilities at the airports, so don't move around much.
NB - my biggest problem - do I put the vagifem in the hand luggage?