Circadian rhythms are endogenous physiological 24-hours rhythms that regulate biological functions in organisms: however, they can be modified by exogenous factors and little is known about the exact mechanisms involved.Hematologic, endocrinological and clinical chemical parameters showed daily fluctuations.Potassium, iron, cortisol, testosterone, somatotropin, prolactin, adrenaline and noradrenaline showed large daily variability. 118 Circadian variations have been observed also for blood parameters, in particular for hematocrit, erythrocyte count and hemoglobin. Hemoglobin, hematocrit, MCV and MCHC depend on time of day with morning maximum and a night minimum. [118][119][120] Hematocrit and hemoglobin relative amplitude in a 24-hour period of observation have been reported to be 2.44% and 3.28% respectively. 119 Similarly, in a study on 7 healthy young men (average age 24 years) and 24 elderly men and women, blood samples were collected at 4 hours intervals during a 24-hour period of observation. All three parameters showed similar variation patterns: the lowest values were found at nighttime, and the highest in the early morning around 8 hours.The percent hemoglobin and hematocrit drop were about 5% in young men and around 13% the elderly groups. In this heterogeneous cohort, hematocrit and hemoglobin levels reached peaks in October and through from March to June. 120 A study on 7685 blood samples collected from British healthy adult men found a variance due to time of day of 1.7% in hemoglobin and 2.1% in hematocrit values: highest mean values were found before 11 am. Authors suggested that they might be related to circadian variations in plasma volume. 121 Authors speculated that rhythmic oscillations in blood parameters could be either intrinsic or due to circadian variations of other parameters such as cortisol. 122 