Latitude-related changes in the amplitude of annual mortality rhythm. The biological equator in man

There is extensive literature describing the effect of season on mortality rates, especially in cardiovascular and respiratory disease. This study com pares latitude with the extent of seasonal variation of monthly deaths from all causes. In developed countries, there is a peak of deaths in winter an d a trough in summer. Monthly numbers of deaths were established in 89 coun tries in the Northern and Southern Hemisphere. Using cosinor analysis, the extent of seasonal variation (amplitude) was established and correlated wit h latitude. The amplitude of seasonality was greatest in mid-latitude aroun d 35 degrees, but low or absent near the equator and subpolar regions. The amplitude can differ at the same latitude. The weather in equatorial region s and in habitations near the Arctic Circle is very different, but death ha s a similar seasonal rhythm. The purpose is to record this epidemiological finding even though no simple explanation is provided. Weather alone cannot explain it, and it is possible that day length (photoperiod) has an import ant, but complex, underlying role.