Chronobiology--derived from chronos (time), bios (life), and logos (study of)--is the objective description of biological time structures and plays an important role in medicine. Circadian rhythms regulate the behaviour, physiology and function of living organisms at many biochemical levels. The influence of the rhythmic manifestation of life may be shown in the constructive effects of growth, development and maturation and is named anachronobiology. In contrast catachronobiology denotes deleterious effects of time and rhythm which may lead to a diseased state. This article summarizes some new data about light as a synchronizer of biological rhythms. A time-keeper--biological clock--is used by the body for readjustment of rhythms when this is desirable. More recently, light has been used to adjust the biological clock following changes of circadian rhythms during shift work or jet lag occurring when flying from one time zone to another. Light may be used to treat depression, sleep disorders, menstrual dysregulations and other illnesses with disturbed circadian and seasonal rhythms. Examples of the importance of chronopharmacology (time-dependent changes in drug metabolism and drug effect) are also presented. New findings indicate that circadian oscillators are under genetic control; a light-influenced regulatory role for cellular immediate-early genes in circadian behaviour has been discovered. This suggests that light is of importance in regulation of macromolecular synthesis at all levels of the circadian system.