In humans numbers of circulating T cells show a circadian rhythm with peak counts during the night and a steep decline in the morning. Sleep per se appears to counter this rhythm by acutely reducing the number of total T cells. The T-cell population, however, is rather heterogeneous comprising various subpopulations with different features and functions and also different circadian rhythms. Therefore, here, we examined whether sleep likewise differentially affects these subsets. We measured eight different T-cell subsets (naïve, central memory, effector memory, and effector CD4⁺ and CD8⁺ T cells) over a 24-hour period under conditions of sustained wakefulness compared to a regular sleep-wake cycle in 14 healthy young men. Sleep reduced the number of all T-cell subsets during nighttime with this effect reaching the p < 0.05 level of significance in all but one subpopulation, i.e., effector CD4⁺ T cells, where it only approached significance. Sleep was furthermore associated with an increase in growth hormone, prolactin, and aldosterone levels, whereas concentrations of catecholamines tended to be lower than during nocturnal wakefulness. The effect of sleep uniformly decreasing the different T-cell subsets is surprising considering their differential function and circadian rhythms, and even more so, since the sleep-induced decreases in these subsets are probably conveyed by different hormonal mediators. Although the reductions in cell numbers are rather small, they are comparable to changes seen for example after vaccination and are therefore likely to be of physiological relevance.