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Unlabelled: Changes in the nyctemeral pattern observed during Ramadan include diurnal fasting from sunrise to sunset and delayed and shortened periods of sleep. The purpose of the present study was to investigate the possible effect of these changes on the circadian rhythm of cortisol and its response to exogenous ACTH., Subjects and Methods: Eleven healthy male volunteers aged 20 to 35 years were studied for two weeks prior to Ramadan. Two short ACTH stimulation tests (250 microg ACTH 1-24 intravenously) at 8 a.m. and 8 p.m. An average 17 days after the beginning of the month of Ramadan, basal cortisol was measured at 8 a.m. and a short stimulation test was performed at 8 p.m., Results: Before Ramadan, cortisol level was higher at 8 a.m. (749.5 +/- 207 nmol/l) than at 8 p.m. (195.18 +/- 79 nmol/l, p<0.001) and the incremental cortisol response to exo genous ACTH was higher at 8 a.m. (peak at 1.167 +/- 46 nmol/l) than at 8 p.m. (peak at 950 nmol/l) (NS). During Ramadan, morning cortisol level (646.3 81 nmol/l) was lower than before Ramadan (NS). The 8 p.m. cortisol level (319 193 nmol/l) increased slightly during Ramadan (p=0.08) but remained lower than the morning level during Ramadan (p=0.001) and before Ramadan (p<0.001). There was not significant difference between cortisol responsiveness to exogenous ACTH at 8 p.m. during Ramadan (peak at 1.102 51 nmol) and at 8 a.m. and 8 p.m. during Ramadan., Discussion: Changes in eating and sleeping schedule was found to reduce morning cortisol level and raise the evening cortisol level. Adrenal sensitiveness to corticotropin stimulation did not appear to be impaired during Ramadan.