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).