This is the first I've ever heard of hypomania, which I often have in conjunction with sleep deprivation. I also had never heard of Bipolar II disorder.

https://en.wikipedia.org/wiki/Bipolar_II_disorder

It's not as dangerous as the kind of mania you get with bipolar 1, but if it occurs to you often, you might want to speak with a doctor. As I understand it, it's difficult to completely control without medical assistance and can have negative effects on your health.

Yes, but these moments were usually followed by an all-nighter. This is my first time hearing of hypomania, but wouldn't the fatigue just be a side-effect of sleep deprivation or the followup of exhaustive mental effort?

Yes. However, one of the side effects of hypomania can be prolonged sleep deprivation/sleeplessness, which creates a pretty nasty feedback loop with days (or more!) of fatigue or outright depression at the end of it.