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This study identified trajectories of morningness–eveningness (M–E) and physical activity when chronological (i.e., time since birth) versus gynecological (i.e., time since menarche) age is used to indicate maturation. Piecewise models were fit for girls (N = 262, ages 11–19) using chronological or gynecological age as the time metric. Girls stayed up later (i.e., eveningness) as they approach menarche. After menarche no change in M–E was observed. In contrast, no change in M–E was detected with chronological age. No change in physical activity was observed before menarche, and physical activity declined after menarche. With chronological age, physical activity declined as girls got older. Gynecological age may be more appropriate than chronological age as a metric for understanding changes in M–E and physical activity.
