You also have to be honest with yourself because discipline requires it. A lot of people make excuses for themselves. They measure their productivity and cite the news blurbs about sleep studies that confirm their bias. They tell themselves and others they can’t. But the reality is that if you’re not sedentary and actually go to sleep with lights and devices off instead of procrastinating you will adjust.
You also have to actually want it enough to deal with annoyance or discomfort. If you convince yourself that you’ll fail you will lay there in the dark for hours until you give in and pull out your phone or laptop. Without motivation you’ll read this and think “I’ve tried all of this, I need some trick or way to solve this that doesn’t require hard work.”