Evolutionary and physiological arguments for the existence of a circalunar clock in humans

Life on Earth evolved in the presence of recurring environmental cycles, namely 12.4-hour tidal cycles, 24-hour day-night cycles, 14.77-day semilunar cycles, 29.5-day lunar cycles, and 1-year annual cycles. In anticipation of predictable conditions associated with these geophysical cycles, organisms have evolved corresponding endogenous time-keeping mechanisms-so-called biological clocks. These clocks oscillate with periods that closely match the periods of the environmental cycles and are therefore called circatidal, circadian, circasemilunar, circalunar, and circannual clocks. While circadian clocks are best studied, and circannual clocks are known to exist in several species, the existence of circatidal, circasemilunar and circalunar clocks is often questioned in terrestrial species. The role of semilunar and lunar rhythms in the reproduction of marine organisms is well known, and they have been shown to rely on circa(semi)lunar clocks in several cases. As life originated in the sea, circa(semi)lunar clocks are likely to be ancient timing mechanisms, which may also be present in terrestrial animals, including humans. Increasing evidence suggests that physiology, metabolism and behaviors including reproduction follow (semi)lunar rhythms in humans and many other animals. Here, we present an overview of (semi)lunar rhythms in animals, with evidence that some of the rhythms are governed by circa(semi)lunar clocks, including those in humans. We compare the properties of circadian clocks with those of circa(semi)lunar clocks, including their entrainment by environmental light cycles - daily light/dark cycles in the former case and monthly full moon/new moon cycles in the latter case, and we examine their putative selective advantages. Finally, we discuss the negative effects of artificial light at night on the entrainment of the putative human circa(semi)lunar clock. Based on all of these observations, we hypothesize that humans possess a circa(semi)lunar clock that was mainly entrained by the environmental moonlight cycles before the introduction of artificial light.