I. Origins of Understanding of the Circadian Timing System and Its Free-Running Clock

The amazing field of chronobiology has recently expanded to include applications of its discoveries to treatment (chronotherapy). Nevertheless, chronobiology has its roots in strict laboratory science.  For many years, studies were conducted on plants (yes!) and animals. Researchers were not even sure that the principles of the science–notably, the existence of an inner clock programmed to cycle at about twenty-four hours–applied to humans.

The seminal experiments were performed with meticulous precision in controlled environments, to create constant conditions isolated from the multiple cues to time of day.

The testing chamber allowed for no changes in temperature, noise level, and humidity. Light was left on constantly, or there was continuous darkness. Lab rats, for example, would exercise on a running wheel far more vigorously during one distinct period during the day (its “nighttime”). But when the data were tracked, the activity bouts cycled at a bit more than twenty-four hours. Hamsters cycled at a bit less than 24 hours. The animals were free running, without any information about the 24-hour day-night cycle outside. However, they would cycle at exactly 24 hours if the test chamber lights were turned on and off once a day.

Of all the possible cycling environmental cues, only light and dark were convincingly shown to adjust inner clock speed to match the earth’s rotational cycle. Even a single brief light pulse presented at a standard time of day could entrain the free-running rhythm of the inner clock.

By the 1990’s, human subjects who lived for weeks in rooms shielded from external time signals exhibited the same inner clock properties as “lower” animals, cycling at an average of about 24 hours 20 minutes, but with very wide inter-individual variations, which we now understand to be largely genetically determined.



  • Richter CP. Biological Clocks in Medicine and Psychiatry. Springfield, Illinois: CC Thomas, 1965.
  • Moore-Ede MC, Sulzman FM, Fuller,  CA. The Clocks That Time Us: Physiology of the Circadian Timing System. Cambridge, Harvard University Press, 1984.
  • Dunlap JC, Loros JJ, DeCoursey P (eds.). Chronobiology: Biological Timekeeping. Sunderland, Massachusetts, Sinauer, 2004.