Menstrual cycle

The menstrual cycle is the roughly monthly hormonal rhythm that prepares the body for a possible pregnancy, counted from the first day of one period to the first day of the next.

A typical cycle lasts 21 to 35 days and has four phases: menstrual, follicular, ovulatory, and luteal. Each is driven by a different balance of estrogen, progesterone, and testosterone.

Because these hormones change your energy, mood, metabolism, and skin week to week, the cycle is sometimes called the infradian rhythm — a body clock longer than a day. Working with it, rather than treating every day the same, is the idea behind cycle syncing.