Sketch map of molecular control of circadian rhythm in mouse and human leukocytes. Mouse and human leukocytes can be stimulated by the physiological stress related to light phase during the day time. Subsequently, p38MAPK/MK2 is activated by phosphorylation. The downstream ROS levels respond differentially to p38MAPK/MK2 activation because of the possible opposite functions of MK2 between mouse and human. ROS positively correlates to the binding capacity of HIF-1α/ARNTL1/CLOCK complex on the promoter of CXCR4, which regulates the expression of CXCR4 and results in the opposite circadian oscillation between mouse and human leukocytes.