A growing body of evidence indicates that cytokines, especially interleukin are involved in the regulation of sleep and wakefulness. The aim of the present pilot study was to investigate the relationship between interleukin-1β (IL-1β) and γ-interferon (γ-IFN) production and the regulation of sleep and wakefulness. Four healthy male volunteers were investigated. After one adaptation night, beginning at 8 a.m. in the morning, the EEG was recorded by means of a mobile long-term EEG and blood samples were drawn every 45 min for the analysis of IL-1β, γ-IFN and cortisol for 24 h. For the analysis of cytokines whole blood cultures were established. After 48 h of incubation in the presence of endotoxin Salmonella typhimurium, IL-1β and γ-IFN levels were measured in the culture supernatants using specific immunodetection assays. Methods of stochastic time series analysis were adopted to evaluate the biochemical data. Our results show the capability of cultered blood cells to produce cytokines upon endotoxin challenge to be at a maximum around the time of sleep onset and during the first hours of sleep, declining during the night to a minimum level in the morning hours. The opposite was observed for cortisol. The analysis of autocorrelation functions gives evidence of a 24-hour rhythm of cortisol and cytokines. The results indicate that the cytokines IL-1β and γ-IFN may play a role in sleep regulation.