The chapter discusses the concepts of exogenous and endogenous components in circadian rhythms, focusing on the mechanisms by which organisms measure and respond to time. The author, Jürgen Aschoff, emphasizes that the term "clock" is misleading as it implies a continuous 24-hour cycle, when in reality, circadian rhythms can have various periods. The key distinction lies in whether the rhythm is driven by internal (endogenous) or external (exogenous) factors. Endogenous components are rhythms that occur naturally within an organism, independent of environmental influences. These rhythms can be disrupted by removing environmental cues, but they persist in constant conditions. The chapter explores how these rhythms are synchronized by exogenous Zeitgeber, such as light and temperature, which act as time-giving agents. The effectiveness of Zeitgeber depends on their periodicity, intensity, and the organism's sensitivity. Exogenous components are environmental factors that can either entrain or mask endogenous rhythms. Zeitgeber can synchronize rhythms, phase-shift them, or influence their frequency. The chapter details experiments to test the effectiveness of Zeitgeber, including phase-shift and frequency variation experiments. It also discusses the role of sensitivity curves and the differential versus proportional effects of Zeitgeber. The author concludes that while light and temperature are the primary Zeitgeber, other factors like feeding schedules and noise can also influence circadian rhythms. The effectiveness of Zeitgeber varies, and the strength of their influence depends on multiple factors, including the organism's sensitivity and the specific conditions under which the experiments are conducted.The chapter discusses the concepts of exogenous and endogenous components in circadian rhythms, focusing on the mechanisms by which organisms measure and respond to time. The author, Jürgen Aschoff, emphasizes that the term "clock" is misleading as it implies a continuous 24-hour cycle, when in reality, circadian rhythms can have various periods. The key distinction lies in whether the rhythm is driven by internal (endogenous) or external (exogenous) factors. Endogenous components are rhythms that occur naturally within an organism, independent of environmental influences. These rhythms can be disrupted by removing environmental cues, but they persist in constant conditions. The chapter explores how these rhythms are synchronized by exogenous Zeitgeber, such as light and temperature, which act as time-giving agents. The effectiveness of Zeitgeber depends on their periodicity, intensity, and the organism's sensitivity. Exogenous components are environmental factors that can either entrain or mask endogenous rhythms. Zeitgeber can synchronize rhythms, phase-shift them, or influence their frequency. The chapter details experiments to test the effectiveness of Zeitgeber, including phase-shift and frequency variation experiments. It also discusses the role of sensitivity curves and the differential versus proportional effects of Zeitgeber. The author concludes that while light and temperature are the primary Zeitgeber, other factors like feeding schedules and noise can also influence circadian rhythms. The effectiveness of Zeitgeber varies, and the strength of their influence depends on multiple factors, including the organism's sensitivity and the specific conditions under which the experiments are conducted.