The daily rhythms of day and night affect nearly all life on Earth. As a result many organisms have evolved rhythmic responses that coincide certain actions with certain times of the day-night cycle. These include a wide range of responses, from behaviour (sleep-wake cycles, feeding rhythms) to physiological rhythms (such as hormonal of visual-sensitivity rhythms) right down to cellular and molecular rhythms (e.g. gene expression and enzyme-activity rhythms). These rhythms are controlled by a “clock” that has an approximate 24 hour period (hence circadian clock, from the Latin circa meaning about, and dies meaning a day).
William Schwartz - Professor of Neurology at the University of Massachusetts Medical School.
What is a biological clock?
There have been many different definition of what a biological clock is:
A biological clock is “an innate physiological system capable of measuring the passage of time in a living organism” (Coleman 1986).
A biological clock is “a widespread cellular mechanism that underlies diverse rhythmic functions in organisms” (Young and Kay, 2001).
Biological clocks are a way for organisms to track the passage of time so that they can anticipate and adapt to changing environmental conditions e.g. plants anticipating the day so that they can prepare their photosynthetic apparatus in time.
Properties of a Biological Clock
Biological clocks have certain properties that define them, irrespective of where they are found:
They continue to run under constant conditions: Biological clocks continue to run in the absence of environmental cues, producing rhythmic patterns. This is a phenomenon called freerunning.
They are temperature compensated: Biological clocks are one of the few biological systems that are not sensitive to temperature. Normally the activity of many systems alters according to temperature e.g. enzymes catalyse reactions at a faster rate at a warm temperature than a cold one. Temperature compensation is important property for the clock, as they have to remain accurate irrespective of environmental temperature changes – otherwise image the problems that would occur on a cold day! Temperature compensation is one of the key defining properties of a biological clock.
They are altered by light: Biological clocks are altered (entrained) by light levels and changing lengths of photoperiod. This allows an organism to anticipate seasonal changes and adjust to changing day lengths.
They are under Genetic Control: Alterations in certain clock genes affect the period length of the clock so that it runs shorter or longer than 24 hours. The genetics of biological clocks is covered in the “cogs” section.
They are adjustable: A biological clock can be reset by external environmental cues (Zeitgebers). These include photoperiod, temperature and social cues. It allows the clock to run with a rhythm most suitable for the organism’s surroundings.
This section covers the "visible" actions of biological clocks such as overt rhythms, entrainment, resetting, and effects such as jet lag.
Rhythms through life
Alterations to the rhythm
This section covers the "nuts and bolts" of the clock - the molecular and cellular mechanisms that make the clock work.
The cogs of a Biological Clock
Links to other useful resources on the World Wide Web