The new time is set the night before, and shifted gradually rather than forced
Waking up earlier is not won at the alarm. By the time it goes off, the decision has mostly been made: how much you slept, how light the room is, and how close the phone is to your hand.
The change that holds is a gradual one. Move the time in small steps, give the body a week to accept each one, and it stops feeling like a fight.
Jumping an hour earlier is a shock, and most people are back where they started within a few days. Fifteen minutes at a time is small enough that the body barely notices, and it adds up quickly: four steps over a month is an hour.
Pick the new wake time first, then move bedtime earlier by the same amount, in the same week. Moving only the alarm just borrows sleep from the night and it comes back as a debt.
Light is the strongest signal your body has for what time it is. Getting bright light soon after waking, ideally natural light, tells it the morning has started, and that is what pulls the whole rhythm earlier over the following days.
The other is what happens in the first ten minutes after the alarm. If the first move is to lie still and negotiate, the new time stays theoretical. If it is to get up and open the curtains, the body reads it as settled.
A shifted wake time usually comes with a few flat days. That is the adjustment, not a sign that it is not working, and it fades. Judging the change on day three is judging it too early.
If you are sleeping badly at the same time as shifting the time, fix the sleep first. Moving a wake time when the nights are already broken just makes both harder.
Missing one morning is not what breaks it. The day after is where routines usually go, because the reaction to one slip is often to abandon the whole thing.
Go back to the new time the next morning, even if the night was bad. One rough day is a rough day. It does not undo the shift.
Fifteen minutes earlier, held for a week, is a real change. Stack four of those and you have the hour, without the week of misery that comes from forcing it all at once.