The average American work day is 8-hours. The average America work week is 5 days. Secret Service agents have been working 12-hours per day. After 8-hours of work, most people are understandably knackered. After all, we are human. To have Secret Service agents doing almost double those hours per work day, on a dangerous, risky job that requires one to be on high alert, is simply too much to ask. You are asking men and women to hold their concentration at a high level for 12-hours per day. That is mentally taxing and draining. It must be effecting their heart, creating anxiety issues, mental fatigue and problems with their nerves.
A study needs to be done on the mental and physical impact long shifts have on agents, who are charged with protecting heads of state and their families. I am certain agents' health is being negatively effected and that can't be good for national security. Shorter shifts, a one hour break per work day, at least one day off per week and an appropriate amount of vacation time each year, would be beneficial.
Side Bar: Heads of state need fresh, well rested and alert people for those types of jobs or, to put it in street terms, that's how you get shot.