the cyanobacterial circadian clock temporally regulates glycogen synthesis and degradation to ensure that the energy storage and utilization take place at the right time for survival [2; 3; 4; 5]. Cyanobacteria must run these two biochemically incompatible processes in a timely manner in order to survive, and this provides unique challenges for cyanobacteria in that not only the redox state must be carefully balanced and modulated [6] but also these opposite reactions must take turns. Otherwise, over-reduction leads to the inevitable generation of reactive oxygen species (ROS), which can destroy any cellular components that they come into contact with. It is no surprise that cyanobacteria managed to put the production of an ROS-mediating protein under the control of its circadian clock, since the ROS level peaks along with that of the photosynthetic activity when the influx of electrons becomes especially abundant [7]. On the other hand, overoxidation is most often the result of starvation. A strain of cyanobacteria with a defective clock output pathway quickly perishes at the onset of darkness, and it has been suggested that this may be due to its inability to make the necessary enzymes for breaking down glycogen in a timely manner as it runs out of energy to survive at night [2]. In sum, the cyanobacterial circadian clock controls the essential physiological processes to maximize their chance of survival, carefully maintaining a balanced endogenous redox state and ensuring the expression of proteins to temporally meet the cellular demands. Figure 1: Simplified schematic diagram of photosynthesis and cellular respiration in the thylakoid membrane of cyanobacteria: (**a**) photosynthesis during the day and (**b**) cellular respiration at night. Black arrows represent the transfer of electrons. The transition from day to night causes the plastoquinone (PQ) pool to become acutely oxidized, and conversely, the transition to the day leads to its transient reduction. COX: cytochrome-c oxidase, Cyt b\({}_{6}\)f: cytochrome b\({}_{6}\)f, PQH\({}_{2}\): plastoquinol, PSI/PSII: Photosystem I/II. Synechococcus elongatus PCC 7942 (hereafter, _S. elongatus_) is a freshwater-living strain of cyanobacteria that clearly demonstrates the characteristics of the endogenous timekeeping mechanism. Perhaps, this single-celled prokaryote's resilience can be attributed to its intricately designed circadian clock, which ensures maximum flexibility in its response to environmental cues, as well as robustness such that it can withstand cellular cacophony. In this genetically tractable species, a circadian promoter with luciferase gene was inserted in a neutral site such that its clock-controlled gene expression cycle can be easily detected as the bioluminescence rhythm of dim and bright [8]. Then, some mutants displaying arrhythmic phenotypes paved the way for identifying that _S. elongatus'_ circadian oscillator is composed of three clock genes: _kaiA_, _kaiB_, and _kaiC_[9]. Resembling two stacked donuts, KaiC is a hexameric protein, composed of a CI and CII domain [10]; the CI domain harbors ATPase activity, which oscillates with circadian periodicity [11], and the CII domain undergoes cyclic phosphorylation and dephosphorylation. KaiA promotes its daytime kinase activity, and KaiB induces its nighttime phosphatase activity. The Kai proteins do not require de novo synthesis of proteins to run [12], and more surprisingly, when the oscillator proteins are purified and mixed with adenosine triphosphate (ATP) and magnesium (Mg\({}^{2+}\)) in a test tube, they retain their post-translational activity, which \"ticks\" as long as ATP is present [13; 14]. In the CII domain of KaiC, there is a protruding linear chain (residues 488-497) called the \"A-loop,\" which switches between two conformations: \"buried\" and \"exposed\" [15]. When its position is buried, KaiC undergoes dephosphorylation; conversely, KaiC autophosphorylates when the A-loop is exposed. KaiA's role at this point is to bind the A-loop (hence the name) and make it stay exposed [16].