Sentence:  At mid-career, though, boredom has a more insidious effect. [...] It can be a devastating experience to no longer feel excited by what you do or to feel you're not an integral part of an organization. That's when the <WSD>doldrums</WSD> set in. 
Meaning of the word marked with <WSD> tags:  "Usually preceded by" '''(the)''': a state of apathy or lack of interest; a situation where one feels boredom, ennui, or tedium; a state of listlessness or malaise.