Sleepiness is a common symptom of infectious diseases. However, the peculiarities and causes of impaired vigilance during host defense activation are largely unknown. It has been shown earlier that mild host defense activation by endotoxin does not affect daytime sleepiness and non-rapid eye movement (NREM) sleep in humans. In the present study we investigated the effects of a more intensive stimulation of the host defense by Salmonella abortus equi endotoxin (0.8 ng/kg), administered 12 h following host response priming by granulocyte colony-stimulating factor (300 micrograms s.c.), on daytime sleep and sleepiness in a placebo-controlled design in ten healthy men. Six equidistant polysomnographically monitored naps were scheduled across the day and the time course of subjective sleepiness was assessed. Endotoxin induced prominent increases in rectal temperature, and in the plasma levels of tumor necrosis factor-alpha, interleukin-6, interleukin-1 receptor antagonist, and cortisol. In the first nap, 1 h following endotoxin administration, total sleep time and NREM sleep stage 2 were reduced, whereas wakefulness and sleep onset latency were increased. Following this nap sleepiness transiently increased peaking prior to the second nap. However, this nap and the following ones were not influenced by endotoxin. These results suggest that prominent host defense activation reduces daytime NREM sleep and increases sleepiness. One cause of daytime sleepiness during infections may be prior sleep disruption and this kind of sleepiness may not necessarily be associated with an increased sleep pressure.