Simple SummaryCircadian rhythms promote mammals’ temporal organization as an evolutionary mechanism of adaptation. It is well-known that the physiological status and well-being of domestic animals may be influenced by endogenous and exogenous parameters. The aim of the present study was to evaluate the circadian rhythm of the blood cell count and leukocyte subpopulations in horses maintained in different housing conditions during the four seasons. All hematological parameters and leukocyte cells showed a different trend influenced by housing conditions and seasons. All hematological parameters showed a daily rhythmicity during spring in horses housed in a loose box and paddock. Lymphocytes and neutrophils showed a daily rhythm in horses housed in loose boxes during spring and summer and in paddocks during winter.AbstractThe aim of our study was to analyze circadian rhythm of the hematological profile of horses housed in a loose box and paddock during the different seasons (spring, summer, autumn, and winter). Blood samples were performed every 4 h for 48 consecutive hours. Red blood cells (RBCs), hemoglobin (HGB), hematocrit (HCT), white blood cells (WBCs), platelets (PLTs), and leukocyte subpopulations (neutrophils, basophils, eosinophils, lymphocytes, and monocytes) were analyzed, and, at the same time, environmental conditions were recorded. A statistically significant effect of housing conditions (p< 0.0001) was observed on all hematological values except for WBC during winter and for neutrophils (p< 0.0001) during spring and autumn. A statistically significant effect of season (p< 0.0001) was found for RBC, HCT, and PLT and for all leukocyte cells (p< 0.0001) except for basophils. The single Cosinor method revealed a daily rhythm of hematological parameters during spring in both groups, and a daily rhythm for lymphocytes and neutrophils was observed during spring and summer in horses kept in a loose box and during winter in horses housed in a paddock. Our results revealed that the response of the immune system is regulated by circadian physiology. Knowledge of the periodic temporal structure of mammals should be considered when evaluating animals’ adaptation to temporizations imposed by the environment.