On the long flights where we carry more than two pilots, we have allocated break time away from the cockpit. During those breaks, you can do whatever you like (sleep, watch a film, read a book, etc). I tend to try to sleep on the plane, but I always bring my laptop on trips to work on projects while downroute. Especially on west-coast trips with the 8 hours timezone change, I am usually awake at 2am which is great for being productive!