Population-level analysis of circadian variations in blood cells

Knowledge of circadian rhythms is crucial in laboratory medicine to understand disease pathophysiologies and improve patient diagnoses and treatments. Here, we aim to determine whether circadian variations in blood cells are present at a population level and assess whether these rhythms are consistent with those reported previously in individuals. In a retrospective observational study we employed blood sample data analyzed at a local emergency laboratory over a period of 7 years, including the number of blood cells, time of sample collection, and patient age (between 18 and 85 years). Data showing cell counts outside the normal range were excluded, yielding a dataset consisting of 189,635 individual data points from 164,982 different patients. Records were grouped into 30-min intervals to determine their temporal distribution and average wave. We detected a circadian rhythm in all studied blood cell types (<i>p</i> < 0.01), with all except neutrophils peaking during the resting phase and exhibiting minimum values during the activity phase. Results were confirmed through parametric and nonparametric analyses. To the best of our knowledge, this is the first report of a significant circadian rhythm in blood cells at population level. The observed temporal variations at population level are comparable to those previously described for individuals.