That's a good point.

Sleeper guests' meals are paid for, but you should still tip the server as though you paid for the meal.

Sleeper attendants are usually tipped for good service. I've had one long distance trip where I didn't tip the attendant at all... And that's because I didn't see them for more than four minutes when we first boarded. (That was the trip I was glad I knew how to convert the cabin on my own.)