You have studied the circadian clock in a certain mouse strain by monitoring the physical activity (e.g. wheel-running) of a few mice over an extended period of time. For the first week, you kept the mice under normal 12 hr light /12 hr dark cycles; they were then transferred to constant darkness. You have plotted the average activity of the mice in the following graph. The light and dark conditions are represented with white and gray backgrounds, respectively. The activity in each day is plotted in black in one row of the graph, and consecutive days are arranged from the top to the bottom. The horizontal axis represents the time of the day. Based on these results, what is the most accurate estimate for the free-running period of the circadian clock in this mouse strain?