Chronobiology in endocrinology.

Temporal endocrine structure (TES). It can be defined as a combination of predictable hormonal changes that are time-related. Regarding their frequency, endocrine rhythms may be circadian, ultradian and infradian. In this context, the endocrine circadian time structure (ECTS), that is closely dependent of some areas of the hypothalamus, is of particular interest. Long and short loop feedback link together the various components: central nervous system (CNS), hypothalamus and anterior pituitary with target glands and tissues. The hypothalamic neuropeptides (releasing hormones or factors - RH or RF - or inhibiting hormones or factors IH or IF) presently known are: thyrotropin releasing hormone (TRH); luteinizing releasing hormone (LH RH); prolactin releasing factor (PRF); Prolactin Inhibiting Factor (PIF); Corticotropin Releasing Factor (CRF); Growth Releasing hormone (GH RH). Some general remarks on endocrine rhythms should be noted: the circadian changes in hormones may depend on each other; even an apparently subordinate rhythm should be considered a true independent rhythm; accurate studies have shown that hormonal secretion occurs in all cases according to a rhythmic organization at many levels; these rhythms may not be evident at a first analysis. The hormone secretion is basically pulsating which makes it difficult to draw standard reference values. Although an ECTS is present at the cell level, in organs etc., it is evident that a rhythm hierarchy exists. Hormonal secretion and sleep-wake cycle. Although several reports state that no rhythm is totally dependent on the sleep-wake cycle, from a general point of view the hormone secretion rhythms can be divided in: sleep-dependent rhythms and sleep-independent rhythms. Meal-timing and hormonal secretion. In animals, meal-timing is a powerful synchronizer; however, there are no definitive and conclusive data to prove that meal-timing is a true synchronizer also in humans, although there have been some reports suggesting it. Endocrine rhythms. Data regarding the endocrine rhythms (circadian-ultradian-infradian) of the numerous hormones as GH; prolactin; aspects of temporal pattern of CRF-ACTH-corticosteroid and of hypothalamic - pituitary - thyroids axis; hypothalamic - pituitary - ovaric steroid and testosterone axis are reported. The study of a possible rhythmic pattern of insulin has been approached from many points of view as the basal rhythmicity of insulin; the diurnal variation of efficacy of injected insulin and of insulin responsiveness to insulinogenic stimuli.