There is a model of self-control in the psychology literature which proposes that it’s a limited resource: We have a certain amount which we use up throughout the day, decreasing our ability to exercise it on subsequent tasks, and then, somehow, it rebounds and we start at the beginning again.
I have a feeling that if I looked at the peaks and valleys of my expressiveness it would follow the same trajectory as this model proposes. For the most part I am extremely controlled: I make a conscious effort to remain unemotional and nonreactive. Eventually, though, the self-control keeping this in place erodes, and all of my emotions are expressed in one exhalation.
Then, eventually, the feeling passes: I can compose myself, and the cycle is complete.