Crew Alertness Monitor
GYS, it's called the Crew Alertness Monitor. Generates an EICAS advisory msg if you haven't touched any switches in a certain amount of time. I believe it is 20 mins. If you don't do anything after the EICAS msg, you get an aural warning to wake you up. First a beeper, then a siren for a wake up call.
So they can't have been alseep for all that time. Something else (not?) going on there.