Wheel-running activity rhythms of melatonin-proficient wild-type and <i>Per2</i> deletion mutant mice.

<p>(A, C) Locomotor activity of melatonin-proficient wild-type (<i>Per2</i>+/+) and <i>Per2</i> deletion mutant (<i>Per2</i> m/m) mice in a running wheel and (B, D) their periodogram analyses under constant darkness. (A, C) The bar over the records indicates the light-dark cycles. The mice were transferred from 12L12D to constant darkness (DD). The <i>Per2</i> mutant mice showed shorter free-running rhythms followed by arhythmicity. Periodogram analyses were performed during the last 7 days. The peak above the diagonal line (<0.1%) indicates the significant circadian period (B).