The various rhythms composing the human time structure are found at all levels of organization, from subcellular organelles to the whole organism. They are superimposed on trends like aging, which is often claimed to be related to a loss of the time structure with a decrease of the adaptation capability of various functions of the organism. Rhythm alterations presumably are involved in the processes of cell and organ damage, leading to death of the organism. Whether these rhythm alterations are a cause or a consequence of the aging process in humans is still debated. Evidence for a causative role has been found in plants and in insects. For instance, it has been shown in Drosophila (Pittendrigh and Minis 1972) that transplantation of young adults raised in a 24h light-dark cycle into an environment with a shorter (eg, 21h) or longer (eg, 27h) periodicity significantly shortens their life span. The importance of the problem of aging results from the increasing number of subjects reaching the age of 65 or older thanks to medical progress, rather than from the lengthening of life expectancy. The chronobiologic approach to aging aims to study the plasticity of the organism in relation to daily and seasonal environmental changes in an attempt to improve the conditions and quality of life of elderly subjects. This chronobiologic approach has to be extremely broad and has to consider biochemical, cell, and organ systems, as well as the organism as a whole. It cannot be limited to one frequency, but has to consider the entire human time structure, with a wide range of frequencies extending from milliseconds and minutes to many years, in individuals and in populations (for reviews, see Haus et al. 1989; Brock 1991; Haus and Touitou 1994a, 1994b; Touitou and Haus 1994).

Changes in the human time structure observed during aging can involve all parameters of biologic rhythms and are found in most frequencies studied. In the circadian range, the most prominent changes in the aging organism are seen in the amplitude and, in some instances, in the timing of rhythms. The latter may lead to changes in the time