Living organisms on this planet have adapted to the daily rotation of the earth on its axis. By means of "endogenous circadian clocks" that can be synchronized to the daily and seasonal changes in external time cues, most notably light and temperature. Most people anticipate environmental transitions, perform activities at biologically advantageous times during the day, and undergo characteristic seasonal responses. The effects of transmeridian flight and shift work are stark reminders that although modern technologies can create "cities that never sleep" we cannot escape the recalcitrance of endogenous clocks that regulate much of our physiology and behavior. Recent progress in understanding the molecular mechanisms underlying circadian rhythms has been remarkable. In its most basic form, circadian clocks are comprised of a set of proteins that generate a self-sustaining feedback loop with a free-running period of about 24 hours. One or more of the clock components is acutely sensitive to light, thus it can be synchronized to local time. (Edery, 2000).
…
