Stress
If flying stresses you out—if you're prone to air sickness, or you're just kind of jumpy—this job will drive you out of your gourd. Not only will you have to be on a plane every day, you won't get to watch a movie to take your mind off things. There's no napping if you're tired, you get minimal bathroom breaks, and your biggest meal will be a packet of pretzels—if that. Ugh.
On top of all the regular hassles of flying—no food, crying babies, cramped legs, jet lag, air sickness, smelly seatmates, lousy in-flight movies, and cabin fever—air marshals have to be on high alert at all times. There is literally no rest for you. You have to be alert to the point of paranoid for the entire flight. That's hard enough on a little three-hour flight, but what about a five-hour trip? Eight hours? Twelve? Fifteen? Non-stop flights across oceans are no joke for air marshals.
Fifteen hours of watching sleeping people suspiciously while fighting the urge to sleep and forcing yourself to act normal? If that's not stressful, we don't know what is.