Biological rhythms play a prominent role in the human life cycle. The endogenous rhythms are entrained by the environment and have an astronomical counterpart which is obvious for daily, monthly, and yearly rhythms, and may possibly also be present in weekly rhythms. Circadian rhythms are present in, e.g. testosterone levels, spontaneous birth, strokes, and death from cardiovascular causes. Circaseptan rhythms are present in, e.g. spontaneous birth, 17-ketosteroid levels, myocardial infarctions, and strokes. The relationship of these rhythms with the suprachiasmatic nucleus (SCN) has not yet been established. Circatrigintan rhythms, such as the menstrual cycle, have so far not been associated with the SCN. Circannual rhythms are present in, e.g. mood, suicides, reproduction, birth weight, sleep and season of birth of psychiatric patients. The human SCN shows strong circadian and circannual fluctuations in the number of neurons expressing vasopressin. The vasopressin and VIP cell population of the SCN develop late, i.e. for a major part postnatally. After the age of 50 the amplitudes of circadian and circannual fluctuations of the vasopressin cell numbers are reduced whereas the number of vasopressin expressing neurons decreases after the age of 80 and do so even more and earlier in Alzheimer's disease. Sex differences are present in the shape of the vasopressin subnucleus of the SCN and in the vasoactive intestinal polypeptide (VIP) cell number. The sex differences in the SCN, the doubling of the number of vasopressin neurons in the SCN of homosexual men, and a variety of animal experimental observations indicate that the SCN is involved in sexual behavior and reproduction. The exact role of the SCN in these processes is subject to current research.