The phytochromes are a family of plant photoreceptor proteins that control several adaptive developmental strategies. For example, the phytochromes perceive far-red light (wavelengths between 700 and 800 nm) reflected or scattered from the leaves of nearby vegetation. This provides an early warning of potential shading, and triggers a series of 'shade-avoidance' responses, such as a rapid increase in elongation, by which the plant attempts to overgrow its neighbours. Other, less immediate, responses include accelerated flowering and early production of seeds. However, little is known about the molecular events that connect light perception with increased growth in shade avoidance. Here we show that the circadian clock gates this rapid shade-avoidance response. It is most apparent around dusk and is accompanied by altered expression of several genes. One of these rapidly responsive genes encodes a basic helix-loop-helix protein, PIL1, previously shown to interact with the clock protein TOC1 (ref. 4). Furthermore PIL1 and TOC1 are both required for the accelerated growth associated with the shade-avoidance response.