Depends on a lot of factors; you aren't giving nearly enough information here.
Is it paid time, or are they on a break? (More complex considerations would be whether or not they are paid hourly or on salary.)
What is the nature of the job? Are they merely there in a "need a warm body" capacity?
There's no one right answer to this, because a gas station attendant taking a nap (doing the graveyard shift of what is probably his second job) is no big deal, nobody cares - the machine/doorbell wakes his ass up. But a truck driver falling asleep at the wheel is an entirely different story...