Variation in diel melatonin concentration in winter and summer. Melatonin was measured in winter (a) at 6:00, 12:00, 18:00, and 24:00, and in summer (b) at 3:00, 12:00, 21:00 and 24:00. The mid-night and mid-day sampling times were the same in both seasons, but we modified the evening and morning samplings in order to keep the time distance to the respective twilights equal. X-axis represents the time of day at which melatonin samples were taken, but we staggered the symbols to facilitate visualization. Y-axis represents back transformed log melatonin concentration. Black symbols depict birds under dark nights (circles, rural (N = 10); triangles, urban (N = 9)), white symbols depict birds under light-at-night of 0.3 lux (circles, rural (N = 10); triangles, urban (N = 10)). Error bars (solid, rural; dashed, urban) represent back-transformed SEM. Red asterisks depict significant treatment effect (light-at-night vs. dark night) at specific times of day, on birds from the two populations combined.