Not only Captains. There was once a pattern that had the 2 F/O's change at Rome, whereas the Capt. & F/Eng. flew Beirut,Rome, London. The two new F/O's appeared at Rome, one climbed into the right hand seat and started the checks and, after T/off as P3, the other took a pillow and blanket down through the grille below the Nav table, and bedded down in the electronics bay below the flight deck.
Halfway to LHR the grille clanged open and they changed places.
The Capt. looked on in amazement, and said to the F/Eng. - "I don't mind him not talking, but who the fcuk is he ?