That's the thing. It was not noticeable at the time. No heavy breathing, no pain in the legs, no sensation of "this is hard" that you get with most formal workouts. But I have no doubt that my heart rate and adrenaline/cortisol levels had already been elevated, enough so to disrupt the typical sleep cycle.

timing can be a pretty big thing. Do you go up stairs slowly, or quickly? One step at a time or two?

If someone is hustling up a staircase, the output can be quite high, even at the level of the 3rd floor. I notice myself, sometimes, forgetting something and moving quickly in and out of my 3rd floor apartment by stairs, and I'll notice myself breathing hard after just the 3rd floor.

And sometimes I hike the building (6 floors) twice or three times, because it feels nice, and everything is timing. One can get easily get a high-intensity, short-interval fairly-involved full-body effort out of ten flights of stairs.

I note a part of me that wants to be defensive wants to say "or perhaps someone else does not know how to try hard enough to make ten flights a good workout?"

Something about your comment feels dismissive, and this random passer-by didn't like it, but I could certainly be projecting, etc etc.