Of course, they calm down by the end of the day (bitchiness takes energy, you know), but it's too late; they've ruined everyone's mood and damaged the morale and productivity of the entire office, all while managing to boost their own.
Scientists have long known that shitty moods make you focus more on grunt work, while good moods allow your mind to wander and push the boundaries of your own creativity, but only recently was it discovered that sliding from the former to the latter over the course of the day improves creativity the most. It actually makes a lot of sense: The morning dickishness stresses you out and forces your brain to prioritize, while the afternoon delight (hee hee) affords you the mental freedom to find the best options. And if you work really hard, you can slide right from "intolerable assholery" to "insufferable cheerability" while skipping right over all the tolerable moods in between, because "manners" and "friendliness" are the flyover states of human interaction.
I'm not sure which direction this metaphor goes anymore.