"In nocturnal rodents, lymphocyte counts in the blood are high during the day, which corresponds to the resting phase of these animals. More specifically, CD4+ and CD8+ T cell numbers in the mouse blood are highest at zeitgeber time (ZT)4–5 (beginning of the day) and lowest at ZT16 (beginning of the night) (20, 21)."
https://journals.aai.org/jimmunol/article/210/1/12/237594/Regulation-of-Cytotoxic-CD8-T-Cells-by-the