Life is a cyclical chemical process that is regulated in four dimensions. We distinguish parts of the cycle: development describes the changes from single cell to adult, and aging the changes from adult to death. Birth to death, a cycle, and there are cycles within cycles—circannual rhythms, menstrual cycles, semilunar cycles, and daily 24 hr or circadian cycles.

Twice a year we get a reminder of the importance of our internal circadian biological clocks. Daylight savings: in October we fall back just an hour, and yet we wake up an hour early on Monday anyway and think meals are late—but only for a day, until our clocks are reset. The reminder is about the way we process environmental information and time, namely that we use external time cues (light and temperature changes that track the day without) to set an internal clock that guides the day within. This internal clock is the lens through which we survey acute external factors; it takes the lead in determining what we perceive as time.