I flew once with a transatlantic crew who had a simple but effective system. When one of them was 'napping' (with the agreement of the other crew member), a small and cheap clockwork kitchen timer was set by the other crew member for 10 minutes. This was then reset on a rolling basis before it reached the 10 minute countdown until the other crew member had completed thier 40 winks. It meant that even if for some reason both crew members immediately went in to napping mode, the maximum time period would be 10 minutes before they would be alerted. Seemed a simple solution, but no doubt the CAA would class it as a major modification to the aircraft if asked to endorse it as a SOP
