Protecthehornet mentions that there is a substantial bar on the door, and that even to let a flight attendant in with coffee or other stimulants, one guy has to get up to let her in.
The rest of the time are you just stuck in the office? I know on very long flights with extra crew there are bunks available for rest. Is it no longer permitted to have one of the flight crew take a walk and greet the pax? Walking around helps us to stay awake in the back and keep the circulation going.
And of course there is another resource, those paying for the ride have an interest in safe arrival. Do UAL still let us listen in to channel 9? I take great interest in all RT, and keep an eye on that cute little moving map on the back of the seat. If I noticed a peculiar deviation from track, if the chaps up front didn't explain nicely, would certainly ask flight attendants if there is a problem.
Passengers are an underutilised resource, many of us also have flying experience. Even a back seat passenger can sound the alarm if the driver dozes off.