[The chronobiology of aging: changes in the temporal-periodic order]

Within an organism the sequence of events and processes during development, maturation, and aging is determined by endogenous and exogenous factors. The endogenous factors consist of oscillatory as well as of unidirectional mechanisms. An individual temporal order ("Eigenzeit") is the result of coupling between various oscillators, interactions between oscillators, and unidirectional processes and of stochastic events. Age-dependent changes of the diurnal rhythms in man and mammals serve as an example for age-dependent changes of the individual temporal order. The parameter of the endogenous (= circadian) diurnal rhythms are changed during embryogenesis, postnatal development, adolescence and aging: amplitude and phase delay increase during development and decrease with age. A higher frequency (ultradian) component predominates in childhood and old age. From experiments with model oscillators, lesioned central clocks in the hypothalamus of mammals and with an arrhythmic mutant of Drosophila, it is concluded that the changes of the temporal order are due to changes in the coupling properties of circadian and higher frequency oscillators. This may be true for the coupling between exogenous periodicities ("Zeitgeber") and endogenous clocks, their mutual interaction and their coordinative function. At present, it is not known whether these changes have consequences for human well-being and health. This may be the case, however, as concluded from indirect evidence.