The seats are cramped for me and I'm 5' 6" .

Although you could probably take a sleeping pill of some kind (and a doctor could prescribe one or advise you of appropriate OTC variety), you might be better off trying for an exit row or bulkhead as they have more leg room. You can request these when checking in and/or request a change in seat assignment when you arrive at the gate. Some airlines hold these seats until the day of the flight, or allow only Elite/Premier/Gold/Silver members to reserve the exit row, but sometimes the seats are still available.

Also the human body isn't supposed to be completely still for several hours as movement is a necessary part of good blood circulation so when confined to a small area it is best to get up, stretch and walk around every 2 hours or so and perform exercises while seated (dorsiflexion/plantar flexion, knee flexion/extension, shoulder rolls, etc.).

evilflierwoman