Drosophila ezoana uses morning and evening oscillators to adjust its rhythmic activity to different daylengths but only the morning oscillator to measure night length for photoperiodic responses.

Animals living at high latitudes are exposed to prominent seasonal changes to which they need to adapt to survive. By applying Zeitgeber cycles of different periods and photoperiods we show here that high-latitude D. ezoana flies possess evening oscillators and highly damped morning oscillators that help them adapting their activity rhythms to long photoperiods. In addition, the damped morning oscillators are involved in timing diapause. The flies measure night length and use external coincidence for timing diapause. We discuss the clock protein TIMELESS (d-TIM) as the molecular correlate and the small ventrolateral clock neurons (s-LN_vs) as the anatomical correlates of the components measuring night length.