Colin S. Pittendrigh discusses the circadian rhythms and their organization in living systems. He emphasizes the need for bold and explicit theory formation in the study of daily rhythms, noting that many observations are fascinating but may not lead to analytic progress. He argues that theory is essential for understanding circadian rhythms, even though it is not always clear which facts are significant. He also highlights the importance of distinguishing between different types of circadian rhythms and their physiological implications. Pittendrigh presents the 2-oscillator model for Drosophila eclosion rhythms, which suggests that the rhythm is controlled by two distinct oscillators: a light-sensitive (A-) oscillator and a light-insensitive (B-) oscillator. The B-oscillator is coupled to the A-oscillator and is responsible for temperature compensation. The model explains the observed transients in the eclosion rhythm and the phase shifts caused by light and temperature signals. He also discusses the broader implications of circadian organization, suggesting that living systems are composed of multiple oscillatory subsystems that are not directly coupled to the light regime. These subsystems can exhibit autonomous rhythms and may be affected by environmental factors such as constant light or dark conditions. He notes that constant light can be detrimental to some organisms, leading to aperiodicity and other physiological changes. Pittendrigh also discusses the physiological consequences of aperiodic light regimes and the breakdown of circadian organization. He highlights the importance of maintaining appropriate phasing among oscillatory subsystems and the potential damage caused by a breakdown in this organization. He concludes that the circadian system is a complex network of oscillators that must be maintained through mutual entrainment to ensure proper physiological function.Colin S. Pittendrigh discusses the circadian rhythms and their organization in living systems. He emphasizes the need for bold and explicit theory formation in the study of daily rhythms, noting that many observations are fascinating but may not lead to analytic progress. He argues that theory is essential for understanding circadian rhythms, even though it is not always clear which facts are significant. He also highlights the importance of distinguishing between different types of circadian rhythms and their physiological implications. Pittendrigh presents the 2-oscillator model for Drosophila eclosion rhythms, which suggests that the rhythm is controlled by two distinct oscillators: a light-sensitive (A-) oscillator and a light-insensitive (B-) oscillator. The B-oscillator is coupled to the A-oscillator and is responsible for temperature compensation. The model explains the observed transients in the eclosion rhythm and the phase shifts caused by light and temperature signals. He also discusses the broader implications of circadian organization, suggesting that living systems are composed of multiple oscillatory subsystems that are not directly coupled to the light regime. These subsystems can exhibit autonomous rhythms and may be affected by environmental factors such as constant light or dark conditions. He notes that constant light can be detrimental to some organisms, leading to aperiodicity and other physiological changes. Pittendrigh also discusses the physiological consequences of aperiodic light regimes and the breakdown of circadian organization. He highlights the importance of maintaining appropriate phasing among oscillatory subsystems and the potential damage caused by a breakdown in this organization. He concludes that the circadian system is a complex network of oscillators that must be maintained through mutual entrainment to ensure proper physiological function.