sentences:  However, like CLOCK-deficient mice in the CLOCK/BMAL1 TTFL, "Npas2"-mutant mice (in the NPAS2/BMAL1 TTFL) still have small defects in their circadian rhythms such as a shortened circadian period and an altered response to changes in the typical light-dark cycle.In addition, "Npas2" knockout mice show sleep disturbances and have decreased expression of "mPer2" in their forebrains.Mice without functional alleles of both "Clock" and "Npas2" became arrhythmic once placed in constant darkness, suggesting that both genes have overlapping roles in maintaining circadian rhythms.