Speaking as an ignorant MOP, surely on a loooong flight, on autopilot, when nothing has to be done and you have audible alarms for any problems then this is not a big issue. If it is a bit of a problem then I would think perhaps more (or regular) alarms for routine checks would be a cheap and simple solution. There may be things that need doing/checking on such flights that I am not aware of but then I doubt either pilot would have nodded off anyway if that were the case.