biological clock

Regular internal rhythm of activity, produced by periodic changes in the concentrations of certain molecules, described as circadian rhythms. Such clocks are known to exist in almost all animals, and also in many plants, fungi, and unicellular organisms. In higher organisms, there appears to be a series of clocks of graded importance. For example, although body temperature and activity cycles in human beings are normally ‘set’ to 24 hours, the two cycles may vary independently, showing that two clock mechanisms are involved.

The first biological clock gene in plants was isolated in 1995 by a US team of researchers.

Exposing humans to bright light can change the biological clock and help, for example, people suffering from seasonal affective disorder.