I'm only a wannabe at the moment, but I currently work as Navigating Officer (keeping 4 hours on, 8 hours off bridge watches) in commercial shipping. I thought the seagoing fatigue guidelines might be interesting for comparison:
Minimum 10 hours rest in any 24 hour period, with rest in not more than 2 periods, one of which to be a minimum of 6 hours.
Minimum 77 hours rest per week.
The regulations are more in-depth than that, but that's the basic outline of rest requirements for watchkeepers. These guidelines are also based on the knowledge that your bed is never more than a few decks away from you, and on a job that requires minimal fine motor skills.
It also requires far lower instantaneous levels of concentration for shorter time periods (ie a maximum of 4 hours, possibly 6 on rare occasions on watch).