Every organism under the sun lives by day and night with a constant cycle of ∼24 hours. Plants, in particular, during the day, convert sunlight, water, and carbon dioxide into carbohydrates and eventually biomass, and emit oxygen as a byproduct of photosynthesis. At night, plants store, transport, and use the carbohydrates, and release energy, carbon dioxide, and water as a byproduct of respiration. Moreover, the temperature and growth conditions change during day and night. These rhythmic cycles are known as the circadian clock, which is derived from the Latin words “circa” (about) and “dies” (day) []. The scientific literature on circadian rhythms began with the daily leaf movements of heliotrope plants even in continuous darkness [], suggesting an internal circadian rhythm. (a) Internal time keepers or circadian clock regulators include CCA1, LHY and TOC1 in a major negative feedback loop (Loop I) of the circadian oscillator in Arabidopsis, which produces a self-sustaining and constant periodicity of 24 hours, even when plants are grown under constant light and temperature. CCA1 Hiking Expedition (CHE) has recently been shown to be a negative regulator of CCA1 []. In addition to CCA1, LHY, and TOC1, other regulatory loops include one (Loop III) consisting of PSEUDO-RESPONSE REGULATOR (PRR) 7 and 9, another (Loop II) of GI and unknown protein, and another (Loop IV) of ZEITLUPE (ZTL), GI, and PRR3. (b) Diagram of CCA1 and LHY (red line) and TOC1 (green line) expression rhythms in a 24-hour clock with 16 hours of light (open bar) and 8 hours of darkness (filled bar). Zeitgeber (ZT) is German for time giver, and dawn is defined as ZT0. Period is the time for completing one cycle of rhythms and is shown from one peak to another (or form one trough to another). The expression amplitude of rhythm is defined as one-half the distance between the peak and trough. Many aspects of plant physiology, metabolism and development are under circadian control, and a large proportion of transcriptome (from 15% up to ∼90%) shows circadian regulation [, ]. For further information, see the many excellent reviews in the field, including historical perspectives of circadian rhythms [], how plants tell time [], regulation of output from the circadian clock [], and the most recent reviews of circadian systems in higher plants [, ].