Biological rhythms, including cycles related to sleep and wakefulness, mood, and cognitive performance, are synchronized with the body's internal circadian clock.[10] The best way to observe the workings of this clock is to experimentally deprive individuals of external cues like light and social interaction and allow the body to experience a "free-running" environment – that is, one in which there are no zeitgebers to influence the body's rhythms.[10] Under these circumstances, the circadian clock alone modulates the body's biological rhythms.[10] Normally however, external cues like light-dark cycles and social interactions also exert an influence on the body's rhythms. These zeitgebers do so by alerting individuals to changes in the likelihood of possible rewards or threats in the environment. For example, humans are more likely to find food and shelter in the daytime and less likely to detect predators in the nighttime, meaning wakefulness tends to be most fruitful during the day and sleep is the safest activity for the nighttime.[11] Therefore, changes in light and darkness influence the body to rise during the day and become fatigued at night.