It has long been known that organisms exhibit a multiplicity of physiological and behavioral rhythms that recur every 24 h. This in turn gave rise to speculation over the existence of a biological clock able to “tell the time” from the ambient day/night cycle. In the 1950s, the term “circadian” was coined to denote these daily cycles, and clock theory was consolidated (1). Experimental data and mathematical modeling proposed that an oscillator generates a characteristic sine-wave output with a regular cycle length, or period, responsible for repeating 24-h rhythms. Crucially, the clock is endogenous, not reactive. It does not merely passively respond to environmental changes but sustains free-running cycles that persist even when organisms are housed in constant darkness in deprivation of external time cues. Such time cues are called Zeitgebers, and by a process of phase-shifting they may reset or entrain the clock to a new environmental rhythm. The master Zeitgeber is light. The out...