Biorhythms, in the popular sense, are supposed regular changes in how fast body processes run — cycles of physical strength, feelings, and thinking — that some people believe you can chart from the day you were born, and that these charts can tell you why you feel high or low. The idea sold well in the late 1900s: books, calculators, later apps. Though actually, careful science has not shown that those three numbered cycles predict your days. What the body really has are clocks — sleep and wake, hormones, temperature — and those are real. That better-supported idea is usually called the body clock, not “biorhythms” in this magazine sense. So the word sits in popular health talk, not in most doctors’ mouths. It is often used in the plural. Be kind to yourself if a chart says you should have a terrible day; the chart is a belief, not a lab result.

生理節律（不是「生理週期」——台灣說生理週期多半指月經；也和「生理時鐘」有別）