Caught in a vice
Its not so much what youre doing as your perceptions of what youre doing. Many middle level managers can make decisions, but they dont have the control to implement them or, you know, to carry them out.
Middle managers have to contend with their superiors, fend off competitive and cut-throat peers wrangling for a seat at the top, and deal with the people problems of their subordinates. OF COURSE they're stressed!
Upper managers are less stressed because they still have their perks, bonuses, and golden parachutes to save them from themselves.