Considering the CKO cellular clock’s shorter intrinsic period, as well as the profound robustness conferred upon SCN timekeeping by interneuronal coupling (Yamaguchi et al, 2003; Welsh et al, 2010;), it seems plausible that 24‐h cycles may simply lie outside the range of circadian entrainment for CKO SCN in vivo, similar to the tau mutant hamster and humans with familial advanced sleep phase syndrome (Ptáček et al, 2007; Meng et al, 2008). This evidence concerns the gene CLOCK and advanced sleep phase syndrome.