Circadian rhythms are roughly 24-hour cycles of biochemical, physiological,
and behavioral processes occurring in various organisms across
life (Bell-Pedersen et al., 2005). Circadian rhythms improve fitness of
organisms in both constant and changing environments (Paranjpe and
Sharma, 2005). A circadian clock system consists of three principal divisions:
sensors of input signals, a central oscillator, and the output pathways
(Lakin-Thomas, 2000). In most cases, the input pathways include
photoreceptor proteins, which entrain the central oscillator (Takahashi
et al., 1993). The central oscillators are formed by autoregulatory transcriptional
feedback loops, which maintain the rhythmic output in
the absence of input signals (Wenderoth and Bock, 1999). The output
pathways transfer the circadian rhythms onto downstream genes.