Circadian clocks drive rhythmic physiology and metabolism to optimize plant growth and performance under daily environmental fluctuations caused by the rotation of the planet. Photosynthesis is a key metabolic process that must be appropriately timed to the light-dark cycle. The circadian clock contributes to the regulation of photosynthesis, and in turn the daily accumulation of sugars from photosynthesis also feeds back to regulate the circadian oscillator. We have previously shown that GIGANTEA (GI) is required to sustain Suc-dependent circadian rhythms in darkness. The mechanism by which Suc affects the circadian oscillator in a GI-dependent manner was unknown. Here, we identify that Suc sustains rhythms in the dark by stabilizing GI protein, dependent on the F-box protein ZEITLUPE, and implicate CONSTITUTIVE TRIPLE RESPONSE1 (CTR1), a negative regulator of ethylene signaling. Our identification of a role for CTR1 in the response to Suc prompted a reinvestigation of the effects of ethylene on the circadian oscillator. We demonstrate that ethylene shortens the circadian period, conditional on the effects of Suc and requiring GI These findings reveal that Suc affects the stability of circadian oscillator proteins and can mask the effects of ethylene on the circadian system, identifying novel molecular pathways for input of sugar to the Arabidopsis (Arabidopsis thaliana) circadian network.