There is increasing evidence that the circadian clock is a signif- icant driver of photosynthesis that becomes apparent when environmental cues are experimentally held constant. We studied whether the composition of photosynthetic pigments is under circadian regulation, and whether pigment oscillations lead to rhythmic changes in photochemical efficiency. To address these questions, we maintained canopies of bean and cotton, after an entrainment phase, under constant (light or darkness) conditions for 30–48 h. Photosynthesis and quantum yield peaked at subjective noon, and non-photochemical quenching peaked at night. These oscillations were not associ- ated with parallel changes in carbohydrate content or xantho- phyll cycle activity. W e observed robust oscillations of Chl a/b during constant light in both species, and also under constant darkness in bean, peaking when it would have been night dur- ing the entrainment (subjective nights). These oscillations could be attributed to the synthesis and/or degradation of tri- meric light-harvesting complex II (reflected by the rhythmic changes in Chl a/b), with the antenna size minimal at night and maximal around subjective noon. Considering together the oscillations of pigments and photochemistry, the observed pattern of changes is counterintuitive if we assume that the plant strategy is to avoid photodamage, but consistent with a strategy where non-stressed plants maximize photosynthesis.