“Musica universalis” is an ancient philosophical concept that claims the movements of celestial bodies follow mathematical equations and resonate to produce an inaudible harmony of music, and the harmonious sounds that humans make are merely an approximation of this larger harmony of the universe. Besides music, electromagnetic waves like light and electronic signals also are presented as harmonic resonances. Despite the seemingly universal theme of harmonic resonance in various disciplines, it was not until very recently that the same harmonic resonance was discovered to also exist in biological systems. Intriguingly, contrary to the traditional belief that a biological system is either at constant homeostasis (not rhythmic) or cycling with a single frequency, it is now appreciated that most biological systems have no homeostatic “set point” but rather oscillate as composite rhythms composed of a series of superimposed oscillations. These oscillations often cycle at different harmonics of the circadian rhythm (approximately 24 hours), and among them the approximate 12-hour oscillations are found to be very prevalent. In this talk, Zhu will focus on these 12-hour oscillations, with special interest in their evolutionary origin, regulation, and functions in mammals, as well as their relationship with the 24-hour circadian rhythm.