Synopsis. Numbers of circulating T cell subpopulations show opposing rhythms with a nighttime peak (represented by naive CD4⁺ T cells; red cosine curve) or daytime peak (effector CD8⁺ T cells; blue cosine curve). Circadian rhythms peaking at night are controlled through the release of cortisol, which strongly increases during the early hours of daytime and via GR activation (with a delay of 3 hours) redistributes T cells from the circulation to bone marrow. Rhythms peaking during daytime are controlled by release of catecholamines, which is increased during daytime, and via β-AR activation leads to an immediate demargination of the cells from the vascular endothelium. Cortisol-sensitive T cells are characterized by high CXCR4 expression, whereas epinephrine-sensitive cells show highest CX3CR1 expression. The 2 chemokine receptors mark 2 distinct T cell populations with only few double-positive cells (conture plots for CXCR4 and CX3CR1 expression on CD4⁺ [top] and CD8⁺ T cells [bottom]). CXCR4 mediates homing of T cells to bone marrow with cortisol supporting this traffic by up-regulating CXCR4. Available data on CX3CR1 tempt to speculate that this molecule facilitates margination of T cells to vascular endothelium, with epinephrine inducing demargination by suppressing adhesive fractalkine signaling.