['Circadian rhythms play an important role in modulating cellular immune responses. The present study was performed to characterise circadian variations in lymphocyte numbers and antigen-specific T-cell functionality in healthy individuals under physiological conditions.', 'Blood leukocyte populations of six healthy volunteers were quantified over 24\xa0h. In addition, antigen-specific T-cell functionality was analysed directly ex vivo from whole blood using flow cytometry based on intracellular cytokine induction after a 6-hour stimulation with adenovirus antigen and Staphylococcus aureus enterotoxin B (SEB), respectively.', 'T-cell numbers and reactivity were stable during daytime, whereas a significant increase was observed during late evening and early morning hours. The percentage of T cells reacting towards adenovirus antigen and SEB showed a 1.76\u2009±\u20090.55-fold (p\u2009=\u20090.0002) and a 1.42\u2009±\u20090.33-fold (p\u2009=\u20090.0002) increase, respectively. Dynamics in T-cell reactivity were independent of the mode of antigen stimulation and inversely correlated with plasma levels of endogenous cortisol. Interestingly, peak frequencies of reactive T cells occurred late in the evening and did not directly coincide with peak numbers of bulk T cells that were observed in the early morning hours.'] Do t-cell numbers and antigen-specific T-cell function follow different circadian rhythms?