When I get on a long flight the first thing I do is take my pants off. That is, I change into pajamas. If I’m flying first class the airline usually gives me a pair (to keep, though my home is full of airline pajamas I rarely taken them along anymore).
If I’m flying business class some airlines provide pajamas, others don’t. United offers them on flights of 12 hours or more but only stocks enough for 60% of passengers to take them.