Polyfunctional CD4 or CD8 T cells are proposed to represent a correlate of immune control for persistent viruses as well as for vaccine mediated protection against infection. A well-suited methodology to study complex functional phenotypes of antiviral T cells is the combined staining of intracellular cytokines and phenotypic marker expression using polychromatic flow cytometry. In this study we analyzed the effect of an overnight resting period at 37°C on the quantity and functionality of HIV-1, EBV, CMV, HBV and HCV specific CD4 and CD8 T-cell responses in a cohort of 21 individuals. We quantified total antigen specific T cells by multimer staining and used 10-color intracellular cytokine staining (ICS) to determine IFNγ, TNFα, IL2 and MIP1β production. After an overnight resting significantly higher numbers of functionally active T cells were detectable by ICS for all tested antigen specificities, whereas the total number of antigen specific T cells determined by multimer staining remained unchanged. Overnight resting shifted the quality of T-cell responses towards polyfunctionality and increased antigen sensitivity of T cells. Our data suggest that the observed effect is mediated by T cells rather than by antigen presenting cells. We conclude that overnight resting of PBMC prior to ex vivo analysis of antiviral T-cell responses represents an efficient method to increase sensitivity of ICS-based methods and has a prominent impact on the functional phenotype of T cells.