This article is part of the PLOS Biology 20th Anniversary Collection. In almost every paper I write and every talk I give, I emphasize, right at the beginning, the pervasiveness of the circadian clock. I do this as a challenge to the imagination: “pervasive” is vague enough to make most people wonder, “in what sense or dimension?” And the answer here is, “in almost any dimension you can imagine.” How can this be? Circadian clocks are molecular machines that organize daily timing.