Sugars provide stored energy and building blocks for all living cells. Sugars also act as dynamic signals to regulate growth and physiology. However, defining specific sugar signalling pathways can be challenging in the context of photoautrophic metabolism because it is difficult to separate responses to light and sugar. Sugar signals regulate circadian rhythms in plants: exogenous sugars can initiate robust circadian rhythms in dark-grown seedlings; and inhibition of photosynthesis in the light can ad just phase of the circadian oscillator. In this context, we have used transcriptomics and a chemical screen to identify potential sugar signalling pathways. To dissect contributions of sugar and light signals on gene networks in Arabidopsis, we have performed an RNA-Seq time-course in dark-adapted seedlings treated with sucrose in the dark or an inhibitor of photosynthesis in the light. The most significantly-enriched class of transcripts responding early to sugar was ‘response to oxygen-containing compound’, pointing to a potential role for reactive oxygen species (ROS)signalling. From a chemical screen for inhibitors of the clock’s response to sugar, we have identified modifiers of ROS production and signalling. These inhibitors also inhibit promotive effects of sugars on root growth and shoot biomass, suggesting ROS signals might contribute to meaningful sugar signalling pathways.