In the age of algorithmic wellness, a color-coded fitness philosophy called cycle-syncing has swept through social media, promising women a hormonal roadmap to optimal training. Researchers, however, find the scientific ground beneath these viral prescriptions remarkably thin — not because the body's rhythms are irrelevant, but because those rhythms are so deeply individual that no universal grid can honor them. The story here is an old one: the human desire for clear instruction meeting the irreducible complexity of human biology.