Circadian clocks are widely present in organisms due to their role in aligning biological processes with environmental cycles. However, their adaptive value remains debated. This review evaluates the evidence for circadian clocks enhancing fitness, considering both historical and current evolutionary pressures. Fitness is defined as reproductive success, and adaptation refers to traits that improve reproductive success through natural selection. The "just-so" problem highlights the challenge of proving that circadian clocks are adaptive, as many biological traits may have evolved for reasons unrelated to fitness. The original adaptive function of circadian clocks was likely to provide an internal estimate of time, enabling organisms to phase biological events optimally with the 24-h day. Phase angle, the alignment of the biological clock with environmental cycles, is crucial for fitness. Studies show that circadian clocks can enhance fitness by optimizing growth, survival, and reproduction. For example, in cyanobacteria, strains with FRPs matching environmental cycles outcompete others. Similarly, in plants, those with FRPs aligned with environmental cycles show better growth and survival. In mice, mutations affecting FRP lead to reduced longevity when not aligned with environmental cycles. The "competition assay" is a rigorous method to test adaptive fitness, comparing different strains under controlled conditions. In cyanobacteria, strains with FRPs matching environmental cycles outcompete others. In plants, Arabidopsis with FRPs aligned with environmental cycles show better growth. In mice, mutations affecting FRP reduce longevity when not aligned with environmental cycles. These studies suggest that circadian clocks enhance fitness by optimizing biological processes with environmental cycles. However, the adaptive value of circadian clocks may also include intrinsic benefits, such as maintaining internal temporal order, which can be important in constant environments. While some studies support the adaptive value of circadian clocks, others suggest that their benefits may be context-dependent. The review emphasizes the need for rigorous experimental approaches to determine the adaptive significance of circadian clocks, highlighting the importance of considering both extrinsic and intrinsic values.Circadian clocks are widely present in organisms due to their role in aligning biological processes with environmental cycles. However, their adaptive value remains debated. This review evaluates the evidence for circadian clocks enhancing fitness, considering both historical and current evolutionary pressures. Fitness is defined as reproductive success, and adaptation refers to traits that improve reproductive success through natural selection. The "just-so" problem highlights the challenge of proving that circadian clocks are adaptive, as many biological traits may have evolved for reasons unrelated to fitness. The original adaptive function of circadian clocks was likely to provide an internal estimate of time, enabling organisms to phase biological events optimally with the 24-h day. Phase angle, the alignment of the biological clock with environmental cycles, is crucial for fitness. Studies show that circadian clocks can enhance fitness by optimizing growth, survival, and reproduction. For example, in cyanobacteria, strains with FRPs matching environmental cycles outcompete others. Similarly, in plants, those with FRPs aligned with environmental cycles show better growth and survival. In mice, mutations affecting FRP lead to reduced longevity when not aligned with environmental cycles. The "competition assay" is a rigorous method to test adaptive fitness, comparing different strains under controlled conditions. In cyanobacteria, strains with FRPs matching environmental cycles outcompete others. In plants, Arabidopsis with FRPs aligned with environmental cycles show better growth. In mice, mutations affecting FRP reduce longevity when not aligned with environmental cycles. These studies suggest that circadian clocks enhance fitness by optimizing biological processes with environmental cycles. However, the adaptive value of circadian clocks may also include intrinsic benefits, such as maintaining internal temporal order, which can be important in constant environments. While some studies support the adaptive value of circadian clocks, others suggest that their benefits may be context-dependent. The review emphasizes the need for rigorous experimental approaches to determine the adaptive significance of circadian clocks, highlighting the importance of considering both extrinsic and intrinsic values.