Plants perceive environmental signals via the circadian system, a pervasive network that provides a biological timing reference. This internal reference has two benefits; it allows plants to measure daylength and enables appropriate responses to environmental cues at different times of day. Importantly, a light signal at dawn will induce a different response compared to an identical treatment provided at dusk. We use the circadian system primarily as a prism to understand how plants are perceiving and responding to light cues at different times of day.