Learn English – The word for the tipping point. Point of change

Your question already contains the best adjective: nocturnal refers to being active at night and sleeping during the day. I don't know about elegant, but this is certainly the most technically-correct word you're going to find. Unfortunately, it doesn't have a noun form, but that can usually be fixed by rearranging your sentences a bit.

Sally was a night owl who was always late to early-morning meetings.

Sally's nocturnal tendencies made her chronically late to early-morning meetings.

I do want to note that in most contexts, I wouldn't interpret either night owl or nocturnal (when applied to people) as explicitly sleeping during the day and working at night. Both words just imply that, left to her own devices, Sally would prefer to go to bed late and get up late, where "late" could have different definitions for different people. If you need to make it clear that someone sleeps during the day, then either you need to say that ("sleeps during the day") or refer to them working the graveyard shift (which admittedly is not the most elegant of phrases).

One such word (and one commonly used by scientists) is vary.

Like increase and decrease, it is not explicit whether vary is an action performed on the variable, or something that the variable "just does".

You should vary the input voltage and observe the result.