Circannual rhythmicities of cortisol levels in the peripheral plasma of healthy subjects.

Seasonal variations of endocrine function have been extensively studied both in rodents and in man. However, only few studies have attempted to concomitantly evaluate and compare circadian and circannual components by inferential rhythmometric procedures. We have examined [9] circadian profiles (single cosinor: P less than 0.01) of plasma cortisol obtained from samples drawn at 4-h intervals in healthy adult subjects of both sexes (71 males, 120 females, aged 20-54 years) along a 7-year span, from 1974 to 1980. All subjects were routine synchronized. Circulating cortisol was measured by a competitive protein-binding technique and expressed as microgram/dl; intra-assay coefficient of variation was always less than 5%. Both conventional and rhythmometric analysis were used to evaluate seasonal differences of mesor, amplitude and acrophase. The population-mean cosinor analyses revealed statistically significant differences as a function of the time of the year in both the circadian amplitude and acrophase. Examining data year by year revealed in 5 out of 7 annual cycles an apparent tendency to a delayed acrophase and to a reduced amplitude with advancing months towards winter. No significant changes in mesor values were found. Further fitting to a 365.25-day cosine curve of circadian parameters available for different months of an 'average' year yielded significant circannual rhythmicity for acrophase and amplitude; crest-value resulted in spring and autumn, respectively. Our data extend previous observations and are consistent with the existence of circannual variations in plasma cortisol levels in the human. In Italian people, cortisol secretion is conceivably higher in cold months; seasonal changes, on the other hand, appear to be of minor entity of at least as inferred by transverse sampling on a relatively large population. Effects of lighting regimen and of other climatological variables upon the adrenal cycle have to be considered.