Variation in daily cycles of locomotor activity. We used one week of activity data before the start of the melatonin sampling, in both winter (a) and summer (b). X-axis represents time of day, Y-axis represents log-transformed mean (± SEM) activity level (counts per hour). For sample sizes and meaning of colours and symbols see Figure 1. In winter we detected treatment effects over the entire day, in summer only at specific hours (depicted by red asterisks).