Biological rhythms spread over a wide range of frequencies : circadian
, ultradian and infradian. Circannual rhythms belong to this latter do
main as their period average one year +/- 2 months. The seasonal rhyth
ms in man of melatonin, prolactin, hormones of bone metabolism, ACTH a
nd HCG are here reviewed and the questions raised by their characteriz
ation - including methodological problems - their origin and their inc
idence on health and diseases.