Apparently, being a librarian induces more stress than working for the emergency services, driving a 125mph express, or teaching a class of ill-behaved children. The reasons offered...
Librarians complained that there was not enough variety in their work, that they did not have enough control over their careers, and they were not allowed to put their skills to full use. The lack of job satisfaction meant they were more likely than other professions to be absent from work, or to vent their frustration on their families when they got home.
It seems that the main source of stress, in this example, is about a lack of control over the working environment. My concern would be the extent to which workers are given the impression they are in control when in reality they're not.
See Stress? Shhhhh . . . by Alan Hamilton of The Times for more details.