Circadian clocks provide a competitive advantage in an environment that is heavily influenced by the rotation of the Earth(1,2), by driving daily rhythms in behaviour, physiology and metabolism in bacteria, fungi, plants and animals(3,4). Circadian clocks comprise transcription-translation feedback loops, which are entrained by environmental signals such as light and temperature to adjust the phase of rhythms to match the local environment(3). The production of sugars by photosynthesis is a key metabolic output of the circadian clock in plants(2,5). Here we show that these rhythmic, endogenous sugar signals can entrain circadian rhythms in Arabidopsis thaliana by regulating the gene expression of circadian clock components early in the photoperiod, thus defining a 'metabolic dawn'. By inhibiting photosynthesis, we demonstrate that endogenous oscillations in sugar levels provide metabolic feedback to the circadian oscillator through the morning-expressed gene PSEUDO-RESPONSE REGULATOR 7 (PRR7), and we identify that prr7 mutants are insensitive to the effects of sucrose on the circadian period. Thus, photosynthesis has a marked effect on the entrainment and maintenance of robust circadian rhythms in A. thaliana, demonstrating that metabolism has a crucial role in regulation of the circadian clock.