From as early as I can remember until about 14, I was a dangerously deep sleeper. I only sleep-walked a couple times, but there were times when I had extended, apparently-waking conversations with people, then had no recollection of them.
I could sleep through people talking, bouncing on the bed next to me, the TV, people clapping, etc. I would fall asleep any time I was not actively doing something. Sitting in class, sitting in the passenger seat of a car, anything where I wasn't actively thinking.
Around 14, I transitioned into symptoms like you describe here. I slept less heavily (though still very heavily) but I could sleep quite a bit, and in the afternoon, it would be willpower vs. drowsiness.
Around 18, I transitioned into what I now think of as a 26-hour day. Every day, I could not sleep until I was exhausted, which made me chase the clock around the day. This was obviously disruptive to work schedules and other normal social life.
Around 22, I transitioned into a pretty normal sleep schedule.
I don't know what changed in any of these cases.
Another oddity: I am able to regularly just not sleep at all, or sleep very little for days on end. I've been up for 72 hours dozens of times. It's quite clear that my functions decline, but 48 hours is not a big deal and longer is possible if there's some compelling reason.
Possibly related, my mother has had sleep problems here entire adult life, very rarely able to sleep more than 4 hours in a night.
I've never taken modafinil but have considered it quite a bit. I mostly think I shouldn't mess with it because I'm lucky enough to have a relatively normal sleep cycle now.