Beneath the noise of daily life, the human body keeps time in ways medicine is only beginning to recognize. A multi-year study of college students wearing fitness trackers has revealed that most people carry slow, hidden biological rhythms in their resting heart rates—cycles lasting weeks, months, or longer—that rise and fall with a regularity as genuine as the circadian clock, yet far less understood. These infradian rhythms appear to be shaped by some combination of social cues, seasons, and internal mechanisms that science has not yet named, suggesting that what we call randomness in health