Homozygous tau mutant (τss) hamsters show an extremely short (20 h) circadian period (τ) that is attributable to altered enzymatic activity of casein kinase 1ε. It has been proposed that coupling of constituent circadian oscillators is strengthened in τss hamsters, explaining their tendency to show strong resetting after prolonged exposure to constant darkness. To evaluate further the circadian organization of τss hamsters, the authors assessed the extent of shortening of period as an aftereffect of exposure to light:dark cycles whose period (T) is 91% of τ and the ability of constant light to induce splitting. They find that τss hamsters show aftereffects comparable to wild types, indicating that normal CK1ε activity is not required for T cycles to shorten τ. This finding also contradicts the proposal that circadian period is homeostatically conserved. However, the authors find that τss hamsters rarely show splitting in constant light. Furthermore, LL does not induce lengthening of τ or reduction of activity duration (α) in these mutants. The authors' findings support the conclusion that the τ mutation alters the coupling between constituent circadian oscillators.