The health maintenance depends on the preservation of the homeostatic systems, such as nervous, endocrine and immune system,
and a proper communication between them. In this regard, the circadian system, which promotes a better physiological system functions
and thus well being, could be considered part of that homeostatic complex, since the neuroimmunoendocrine system possesses circadian
patterns in most variables, as well as circannual or seasonal variations. With aging, an impairment of the homeostatic systems occurs
and an alteration of circadian system regulation has been demonstrated. In the immune system, several function parameters, which
are good markers of health and of the rate of aging, change not only with age (immunosenescence) but also throughout the day and year.
Indeed, with advancing age there is a modification of immune cell circadian function especially in lymphocytes. Moreover, immune
functions at early afternoon correspond to more aged values than at morning, especially in mature subjects (60-79 years of age). In addition,
these mature men and women showed a significant impaired immune cell function, which is especially remarkable in the winter. It
is noteworthy the role of immunomodulatory hormones, such as melatonin, in the regulation of biological rhythms and their involvement
in the aging process. Furthermore, the evidence of a neuroimmune regulation of the circadian system and its disturbance with aging,
highlights the importance of proinflammatory cytokines in this complex cross-talk. The biological rhythms disruption with age and some
diseases (jet lag, cancer and seasonal affective disorder), could contribute increasing the immune system impairment and consequently
the loss of health.