Growing evidence points towards a beneficial effect of sleep on immune function. Human studies indicate that the T cell mediated adaptive immune function including formation of antigen specific antibodies is facilitated by sleep. Along this line, here we aimed to dissociate the effect of sleep and circadian rhythm on circulating interleukin-7 (IL-7) and interleukin-15 (IL-15). These cytokines play a key role in the homeostatic regulation of naïve and memory T cell numbers and are critical for the differentiation of memory T cells. Serum IL-7 concentration and expression of membrane-bound IL-15 (mIL-15) on CD14+ monocytes were measured in 18 men on two occasions: once during a regular 24-h sleep-wake cycle and another time during a 24-h period of continuous wakefulness. During sleep and especially during late sleep serum IL-7 concentrations were distinctly increased as compared to wakefulness (p < 0.05). mIL-15 density on monocytes remained unchanged by sleep. The sleep-dependent increase in IL-7 concentration was associated with increased REM sleep, but did not correlate with concentrations of GH, cortisol or norepinephrine during sleep. The findings concur with the notion of a supportive influence of sleep on T cell function related to formation of T cell memory.