Circadian plasma corticosterone concentrations (μg/dL) at ZT0, 4, 8, 12, 16, and 20 in rats during postnatal ontogenesis at P1, P3, P6, P12, P14, P16, P21, and P24. Dotted line: mesor; solid line: cosine curve and 95% confidence interval of acrophase. *Differences between peak and mesor concentrations. Black bars represent dark phase.