The ubiquity of circadian rhythms (driven by underlying circadian clocks) in various behavioural and physiological processes across a variety of life forms supports the hypothesis that such rhythms are probably adaptive. This is further substantiated by studies demonstrating that dysfunctional circadian clocks are associated with multiple aberrant physiological processes. However, owing to the complex interplay of life-history traits that collectively mediate realised fitness of organisms, rigorously testing whether circadian clocks are indeed adaptations turns out to be quite challenging. Here, we review our current state of knowledge on the adaptive benefits of circadian clocks, and discuss the pros and cons of various studies, followed by a brief discussion on our recommendations for how improved experimental designs can be employed in future.
Nikhil, K. L., & Sharma, V. K. (2017). On the origin and implications of circadian timekeeping: An evolutionary perspective. In Biological Timekeeping: Clocks, Rhythms and Behaviour (pp. 81–129). Springer (India) Private Ltd. https://doi.org/10.1007/978-81-322-3688-7_5