Complexity and non-linear description of diurnal cortisol and growth hormone secretory patterns before and after sleep deprivation.

OBJECTIVE The circadian secretory profiles of cortisol and growth hormone (hGH) in normal subjects are interrelated. Slight alterations in cortisol secretion are paralleled by similar ones of hGH secretion. Under physiological conditions an inhibitory effect of glucocorticoids on hGH secretion is more potent than a stimulatory one, while in normal young subjects the nychtohemeral cortisol and hGH levels are lower and higher, respectively, post 24 hours total sleep-deprivation, compared to baseline values. The aim of the present work was to further assess the qualitative characteristics of the 24-hour secretory patterns of these two hormones before and after 24 hours total sleep deprivation, by studying their non-linear profiles using fractal analysis. METHODS Cortisol and hGH were measured in 24-hour samples drawn from 10 healthy men (mean age SD: 24 +/- 1 yr, mean BMI SD: 25 +/- 1 kg/m2) before and after 24 hours total sleep deprivation. Twenty-four hour blood sampling was performed serially every 30 min the day before and the day after total sleep deprivation. The 24-hour hormone profiles were analyzed by Fourier spectrum, in order to verify periodicities; the corresponding attractors were drawn and their respective fractal dimensions were calculated using the box counting method. RESULTS Diurnal cortisol levels before sleep deprivation gave rise to a fractal attractor with a D0 fractal dimension of 2.65 +/- 0.03, which decreased, post-sleep deprivation, to D0: 2.18 +/- 0.04. Growth hormone before sleep deprivation gave rise to a fractal attractor with a D0 dimension of 1.96 +/- 0.60, which increased to 2.24 +/- 0.60 post-sleep deprivation. These post-sleep deprivation changes of the fractal dimensions of cortisol and hGH, suggest that sleep deprivation leads to a more regular secretory profile of cortisol, while it tends to render hGH secretory profile less regular. Additionally, these changes of the fractal dimensions parallell the previously described quantitative overall changes of these hormones. CONCLUSIONS The post-sleep deprivation decrease of cortisol fluctuation might reflect the mechanism by which sleep deprivation temporarily improves mood in melancholic depression, a condition associated with hyperactivity of the hypothalamic-pituitary-adrenal axis.