At all times, cycles have focused men's attention and fashioned his life. Today, thanks to genetic, one can find tracks of circadian rhythms programming until cell's DNA, and this in a very amazing and similar manner from amoebas to mammals. A particular rhythm interests the researcher in oncology: the circadian rhythm of melatonin. It stands at the junction of several domains: somatic, immune and psychic, through the many receptors found on leukocytes, through the links between this hormone production and the one of many cytokines but also with activity, life habits and "stress". On an other hand, antioxydant action of melatonin gives a serious argument concerning its possible role in cancer aetiology. As for them, studies on sleep confirm the large ubiquity of biological cycles, for instance thanks to the observation of the impact of particular genetic mutations on advance or delayed sleep syndrome. Because of the great diversity of cyclic phenomena, the study of chronobiology cannot be undertaken today without a wide interdisciplinary collaboration. During the 13th congress of the "Association Francaise de Chronobiologie Medicale", this study has been continued mainly in three different directions of research: fundamental, applied and transverse. Many original experimental results have been presented and new ways of multidisciplinary research specified. The important scientific fecundity of this very convivial annual congress never lacks to satisfy its participants: it continues to favour the onset of new projects, enabling to avoid major shelves thanks to the constructive criticism of each domain specialists.