Circadian rhythms and their application to occupational health and medicine

Detailed measurements of many physiological variables over the course of 24h have indicated that normal human subjects show circadian rhythms. Such rhythmicity derives from an internal clock as well as our rhythmic environment and rhythmic habits. The internal clock is normally adjusted to run with an exact 24-h period by the rhythmic influences in the environment so that, in health, there is a synchrony between the different body rhythms, the internal clock and the external clock. There is evidence that in some individuals such a 'matching' of rhythms does not occur. Further, abnormalities are imposed upon this system after time-zone transition and during irregular hours of work (including night work). In all cases, undesirable side-effects are evident in the individual concerned. Where possible, an explanation of these problems is given together with advice that might help to ameliorate them. When applications to medicine are considered, a knowledge of normal circadian rhythmicity can be a diagnostic aid and can, in addition, indicate the most appropriate time for replacement therapy. It is also apparent that the efficacy of drugs can depend upon their time of administration. These aspects of chronopharmacology have most application in the treatment of asthma, arthritis and cancer, all of which are considered in some detail.