Plants store a part of assimilated carbon into starch during the day, and degrade it into sugars at night. In Arabidopsis, starch accumulation in the day and degradation at night occur almost linearly with the shape of this diel starch profile adaptively changing to allow continuous supply of sugar even in long night conditions. Consumption of starch to almost zero at dawn presumably requires the circadian clock. In addition, recent studies have shown that the clock is entrained by photosynthetically derived sugars. To explore adaptive significance of this bidirectional feedback between the clock and sugar metabolism, we developed a model focusing on the dynamical interaction between carbon metabolism and the clock. First, we showed that linearity of starch availability can be an emergent property of sugar homeostasis. Next, we took into account the phase response of the clock oscillation against sugar signals. We simulated plant growth under various light/dark conditions to examine the impact of the clock entrainment by sugar on biomass accumulation. We found that the entrainment surely improved sugar homeostasis, indicating that it is essential for plants undergoing seasonal change in day length.