Figure 3. Period aftereffects are not altered in Period mutant mice. After gradually changing the length of the T-cycle, mice were maintained in the target T-cycle for 2 weeks and then released into DD. A–D, The free-running periods (mean ± SD h) of the wheel-running activity rhythms following entrainment to T21, T22, T24, and T26 in wild-type (A), Per1−/− (B), Per2−/− (C), and Per3−/− (D) mice were determined by χ2 periodogram for d 1–15 in DD. The period was determined only from mice that entrained to the T-cycle (the number of mice that entrained in each group is indicated in Fig. 2). Per1−/− mice were not assessed in T22 (ND, Not determined).