When weboard a flight, we know that we don't have to work, We may have just finished a day's work. The crew (flight and cabin) get on boar knowing they have to work. So they tend not to doze whilst reading their books. Of course, fatigue is another issue.
I'm self employed and some days, like everyone, I feel rather disinterested in going to work. But I know that the moment I start the car to drive to my client and do what they are paying me to do? I'm wide awake.