In 12:12 hour light:dark conditions, males and females carrying P{dmpi8UP} (in a per⁰¹ background) show significantly reduced daytime sleep levels compared to wild-type controls carrying P{dmpi8WT} (in a per⁰¹ background). The P{dmpi8UP}, per⁰¹ males and females also show reduced levels of sleep compared to controls when exposed to prolonged periods of constant light. The P{dmpi8UP} transgene contains both per and dyw coding sequences (represented in FlyBase by per^M2M1 and dyw^M2M1 respectively); experiments using transgenes in which these coding sequences are individually or simultaneously mutated demonstrate that the reduced sleep phenotypes seen in P{dmpi8UP}, per⁰¹ flies are mediated via dyw function and not per function.