There's a recurring truth in my life: "keep it simple".
I first encountered this phrase during my engineering preparatory days. My mentor, a greybeard whom I still admire and keep in touch with, shared this tidbit of wisdom when I proposed some over-complicated solution to a problem-- as young folks who are looking to prove themselves tend to do.
I don't recall much about that specific problem and its solution, but the principle has stuck with me. This principle doesn't apply to only engineering problems. I've known it to be a truth in almost all facets of my life.
For exercise, just putting on my running shoes in the morning and doing it works well.
For diet, keeping a pantry stocked with healthy foods makes it easy for me to eat well.
For sleep, I've found that a regular schedule with activity in the morning & afternoon and rest in the evening makes falling asleep easier (which I'm unable to follow as of yet)
Chuang Tzu once wrote, "when the shoe fits, the foot is forgotten."
Sometimes it takes a while to find a shoe that fits. Simplicity, for me, is knowing not to go shopping for shoes when your feet are comfortable. :)