Representative double-plotted actograms (A-D) of hamsters in Experiment #1. Animals were pinealectomized, exposed to long (15L:9D) or short (10L:14D) photoperiods and simultaneously infused daily with melatonin for 5 h or 10 h. Following 6–7 weeks of infusions, catheters were disconnected and hamsters were exposed to constant darkness (DD) as indicated along the left margin of the actogram. The light:dark cycle is indicated with white and black bars, respectively, above each actogram, and the time of daily melatonin infusion is denoted with a crosshatched rectangle. Mean (± sem) phase angle of entrainment (time of lights off minus time of activity onset) (E) and activity duration (F) of hamsters in each group. Asterisks indicate a statistically significant effect of photoperiod (*** p < 0.001).