Biological rhythms: Living your life, one half-day at a time

Abstract

Circadian rhythms play a preeminent role in our life, organizing our physiology and behavior on a daily basis to resonate with our fluctuating environment. However, recent studies reveal that hundreds of mouse and human genes are expressed with a 12-h pattern. We take a close look at mammalian 12-h rhythms, their potential mechanisms and functions, and evidence linking them to circatidal rhythms, which enable marine animals to adapt to tides.

Keywords: Circadian mechanisms; Circadian rhythms and sleep.

Fig. 1. Circadian (~24 h) and circatidal (~12.4 h) rhythms coexist in crustaceans.

In P. hawaiensis, interacting circadian and circatidal clocks are entrained by the light/dark and tidal cycles, respectively, to control rhythmic behavior. See main text for details. Created in BioRender. Emery, P. (2025) https://BioRender.com/caeejhf.

Fig. 2. Circadian (~24 h) and circasemidian (~12 h) rhythms coexist in mice.

Both circadian and circasemidian rhythms of gene expression can be observed in mice (in whole animals or cell lines). They might be generated through three, non-mutually exclusive, mechanisms. Left: the circadian clock generates both 24 h and 12 h rhythms. Circasemidian rhythms could be the result of two distinct sets of transcriptional regulators taking turn to promote gene expression every 12 h (double arrow). Middle: the circadian clock collaborates with systemic factors controlled by feeding to generate circasemidian rhythms. Right: a dedicated 12-h oscillator, independent of the circadian clock, generates circasemidian rhythms, reminiscent of the independent circadian and circatidal clocks in crustaceans (see Fig. 1). Created in BioRender. Emery, P. (2025) https://BioRender.com/n22k1pa.