Study Objectives: To determine the association between common genetic variation in the clock gene pathway and objectively measured actigraphic sleep and activity rhythm traits. Design: Genetic association study in two population-based cohorts of elderly participants: the Study of Osteoporotic Fractures (SOF) and the Osteoporotic Fractures in Men (MrOS) study. Setting: Population-based. Participants: SOF participants (n = 1,407, 100% female, mean age 84 years) and MrOS participants (n = 2,527, 100% male, mean age 77 years) with actigraphy and genotype data. Interventions: N/A. Measurements and Results: Common genetic variation in 30 candidate genes was captured using 529 single nucleotide polymorphisms (SNPs). Sleep and activity rhythm traits were objectively measured using wrist actigraphy. In a region of high linkage disequilibrium on chromosome 12p13 containing the candidate gene GNB3, the rs1047776 A allele and the rs2238114 C allele were significantly associated with higher wake after sleep onset (meta-analysis: rs1047776 P-ADD = 2 x 10(-5), rs2238114 P-ADD = 5 x 10(-5)) and lower LRRC23 gene expression (rs1047776:. = -0.22, P = 0.02; rs2238114:. = -0.50, P = 5 x 10(-8)). In MrOS participants, SNPs in ARNTL and NPAS2, genes coding for binding partners, were associated with later sleep and wake onset time (sleep onset time: ARNTL rs3816358 P-2DF = 1 x 10(-4), NPAS2 rs3768984 P-2DF = 5 x 10(-5); wake onset time: rs3816358 P-2DF = 3 x 10(-3), rs3768984 P-2DF = 2 x 10(-4)) and the SNP interaction was significant (sleep onset time P-INT = 0.003, wake onset time P-INT = 0.001). A SNP association in the CLOCK gene replicated in the MrOS cohort, and rs3768984 was associated with sleep duration in a previously reported study. Cluster analysis identified four clusters of genetic associations. Conclusions: These findings support a role for common genetic variation in clock genes in the regulation of inter-related sleep traits in the elderly.