Relationship between melatonin and dawn activity. X-axis represents change in melatonin between midnight and either 6 am (winter, (a)) or 3 am (summer, (b)), Y-axis represents log-transformed mean activity counts in the hour preceding the onset of morning civil twilight (winter: 6:45, summer: 3:45). Dashed (light-at-night) and solid (dark night) black lines represent regression slopes from fitted linear models. Grey shaded areas represent 95% CI. For sample sizes and meaning of colours and symbols see Figure 1.