The central clock is generally thought to provide timing information for rest/activity but not to otherwise participate in regulation of these states. To test the hypothesis that genes that are components of the molecular clock also regulate rest, the authors quantified the duration and intensity of consolidated rest and activity for the four viable Drosophila mutations of the central clock that lead to arrhythmic locomotor behavior and for the pdf mutant that lacks pigment-dispersing factor, an output neuropeptide. Only the cycle (cyc01) and Clock (Clk(Jrk)) mutants had abnormalities that mapped to the mutant locus, namely, decreased consolidated rest and grossly extended periods of activity. All mutants with the exception of the cyc01 fly exhibited a qualitatively normal compensatory rebound after rest deprivation. This abnormal response in cyc01 was sexually dimorphic, being reduced or absent in males and exaggerated in females. Finally, the cyc01 mutation shortened the life span of male flies. These data indicate that cycle regulates rest and life span in male Drosophila.