I think I was in about 4th or 5th grade-- back when we were still living in Denmark-- when I first became consciously aware that I had a difficult time staying awake in some classes.
I do remember the particular sequence of events... how it would happen, each time.
It was often during history class, which I found particularly boring. The teacher would be writing numbers on the board (dates) and droning on and on about some event in 15th century Britain that I really didn't care about. Slowly, I would grow aware of the hardness of my chair... that plain wooden seat... and I would start moving my butt around, a bit. Then there would be the sensation of tiny "pin pricks" on my butt... a little bit like the pins-and-needles sensation when your foot goes to sleep.
I was well aware that I was "supposed to" focus on what was going on in class... but my eyelids just felt so heavy, like I hadn't slept in several days. But I was acutely aware of how embarrassing it would be if I actually fell asleep in class, so I'd move my head to try to move on. I'd look out the window, at the weather; at anything the moved; trees in the breeze. Our "home room" was on the 3rd floor... and faced a road, across from which was the local train station. I'd watch people come and go, getting ready to take the trains. I envied them their freedom.
By this point, the teacher's voice was just a backdrop for my efforts to stay awake... and my wandering mind, which was mostly on memories of taking the train into the city to visit various family members. History was no longer even part of my awareness.
I'd only look out the window for a while... then I'd "look" back at my desk, or in my history book. I was extremely adept at looking like I was "concentrating" even though what I was looking at was mainly a fog...
From time to time, I'd "snap back" to where we were... in class... teacher talking about some battle.
The "danger zone" came when he finished his talking and started asking questions. I did NOT do well when called on, unless I knew the answer ahead of time.