In the US military, new recruits are put on a very early schedule (think: 5:00am wakeup) and an appropriately early bedtime. Within a matter of weeks, they become habituated to it such that everyone's body automatically starts operating on that schedule, even if no one wakes them up in the morning. I am skeptical that there is a biological reason 20-50% of the population can't get up early because it doesn't seem to be an issue in the myriad environments where waking up early is a general requirement. These environments aren't selecting for people that can get up early, they habituate people into getting up early. It clearly is possible most of the time.
I've had points in life where school or work demanded I wake up early. After a few weeks I could go to bed and sleep at something approximating the time required for an appropriate night of rest and would automatically wake up at the time required - true.
This sounds like success, but is significantly less so than it sounds:
- It didn't change the time my body wants to do those things.
- Waking up: I'd wake up because it's the time I currently wake up daily and know I need to, but the feeling of wanting to go back to bed for hours was still there - not even feeling like I'm not rested, just a vague sense of "this is a time for sleep". Performance for the first 3-5 hours of my day was noticeably poor.
- Going to bed: With enough practice I could usually sleep early enough. I never once wanted to sleep early enough or actually felt tired when I had to go to bed. I had to have multiple alarms set to go to bed, because even though that was the time I'd been going to bed for months - my body still does not give off a single signal to sleep at that time.
- One single night off-schedule would completely blow up the whole thing and take a week or more to feel back on the schedule fully. Months of being on the "early" schedule could be instantly thrown away and I'd be back on the "late" - adaptation is only required in the unnatural direction, the natural one is instant.
- I have actually spent weeks off-grid hiking and away from screens/artificial light before and the behavior still persists. It's not until hours after dark that I feel tired and I don't naturally want to wake up until far past sunrise. (I am also perfectly happy sleeping in full daylight, room light/darkness has no impact on my sleep).
I have the same feeling regarding much of what you said, with the slight difference that even though my body didn't feel the need to go to bed, strangely enough, after lying in bed for 20 minutes or so, I often fall asleep without noticing any sleepiness before that, just wide awake, and the next thing I know I am waking up in the morning.
While I was never in the military, I have been in environments where I needed to be an earlybird. It's exactly as you say: the adaption to the environment makes the early rising palatable. However, once left to my own devices, I quickly return to my nocturnal habits. For me, it's less revenge bedtime procrastination and more so that nights are better for long blocks of uninterrupted time than days.
I think people can train themselves to get up at any time, even in the middle of the night, but I have doubts that they'd all be fully alert at that hour even if they did it for months or years. I can train myself to wake up early, but my brain doesn't fully "wake up" until much later in the day. It might be genetic (https://www.sciencedaily.com/releases/2015/05/150514085748.h...) and there are groups looking at drugs to help night owls adapt to the artificial constraints modern society imposes on them.
I think is a combination of physical activity waking the recruits up and keeping them alert, as well as physical exhaustion helping them fall asleep. I suppose anyone who is chronically unable to stick to that schedule would get weeded out quickly.
