Similar here. (I was never tested for ADD; maybe I should.) And the worst thing for me is having a meeting.

Work at least keeps me somewhat awake. Over years I have developed some coping strategies, for example when I feel sleepy I open a text file and start writing detailed notes about what I am currently doing and why. That way I can continue to work, because when I fall asleep for a moment and lose the current thread of thought, I just read what I wrote recently and continue. Also, the act of typing, and seeing changes on the screen help to keep me awake.

But once in a while we have a long meeting, often in dark-ish and not well ventilated place, and that is like feeding me sleep pills.

I don't have ADD and it happens to me every day.

I tried to hint a doctor once to maybe test me for this, but she wouldn't give. I feel like adult ADD isn't yet recognized in my country as a legitimate thing.

Check your diet. This happened to me when I was consuming large amounts of carbs for breakfast and lunch. I switched to keto and noticed the tiredness went away in the afternoon and I didn't require 3+ cups of coffee in the morning. Basically I was crashing multiple times a day which was driving me to the point of falling asleep at work or behind the wheel.