It’s never just one thing.
If you start an exercise program, but never improve poor eat habits, and sleep poorly, it’ll take longer (if ever) to see results.
If you start yoga and meditating, but aren’t willing to remove yourself from other stressful situations (work, home, friends, family), it’ll take a long time to see the benefits of your hard work.
If you got the black out curtains, put your phone down hours prior, and got the most fancy “sleep machine”, however you don’t exercise or move much, and eat poorly, you’re still not going to sleep well.
Fortunately, our body isn’t as simple as how you optimize a car or computer.
All of our systems are working together, and putting all the focus on 1, and little regard to the other very often will lead to frustrations and difficult reaching your goals.