The effects of various space scenarios on the circadian system and alertness were investigated. The study of the circadian system under various conditions is important because core body temperature and many other physiological parameters (e.g., hormone levels) exhibit prominent circadian variation. The circadian system has a direct influence on the sleep-wake behavior and thus on performance. Alertness is an easily accessible subjective parameter which is strongly linked to cognitive performance. Simulation of alertness for different sleep-wake schedules may help to delineate crucial phases during missions and to plan counteractions. Although the level of alertness can be accounted for by the combination of circadian and homeostatic processes, additional factors such as motivation or excitement play a role.