They're tired at the end of a day of shooting, no doubt. The men have it a bit harder, as it's harder to get it up, and cum on command. You just have to get used to do it. Like any physical job, it will wear you out.
Most of them have a good physical condition, working out, training, dieting etc. Of course they get tired too and get some rest. Oh and drugs might help a bit
How much do you think they have? Like once a day? Twice a day? Five times? Some people do that without any cameras around. Plus it probably helps if you're getting paid.