Circadian rhythms are in the spotlight now that a molecular basis has provided a mechanism for their existence, the importance of which has just been recognized by the Nobel Assembly at the Karolinska Institutet that awarded the 2017 Nobel Prize in Physiology or Medicine jointly to Jeffrey C. Hall, Michael Rosbash and Michael W. Young “for their discoveries of molecular mechanisms controlling the circadian rhythm”. The term “circadian”, derived from the Latin “circa” (about) and “dies” (day), was coined in 1959 by Franz Halberg to indicate that circadian rhythms had a period of about 24 hours. The meaning of the term refers to free-running and the partly endogenous nature of circadian rhythms, their period differing slightly but statistically significantly from 24 hours in the absence of environmental time cues. It also indicates the statistical uncertainty in estimating the period: even under synchronized conditions, circadian characteristics, including the period, cannot be estimated exactly, because of measurement error and changes occurring from one day to another, as amply documented for blood pressure. In humans, the lighting regimen serves as the primary synchronizer of circadian rhythms. The phase of circadian rhythms can be inverted by reversing the lighting regimen. In addition to light, the feeding schedule is another important synchronizer of circadian rhythms. Under conditions of time-restricted feeding, the phase of certain variables related to metabolism can be greatly affected by the time when food is made available, whereas that of other variables such as cortisol remains practically unchanged. As shown by Halberg, circadian rhythms are ubiquitous; they are found across species, from archaea  to humans. They characterize most, if not all, variables, such as activity, temperature, blood pressure, heart rate, respiration, hormones and enzymatic activities. Apart from spontaneous rhythms, the response to a given stimulus is also predictable, depending on the circadian stage when it is administered. Response rhythms are the fundamental principle underlying chronotherapy.