Objective: Circadian rhythms influence the timing of behavior, neurological diseases, and even death. Rare mutations in homologs of evolutionarily conserved clock genes are found in select pedigrees with extreme sleep timing, and there is suggestive evidence that certain common polymorphisms may be associated with self-reported day/night preference. However, no common polymorphism has been associated with the timing of directly observed human behavioral rhythms or other physiological markers of circadian timing at the population level. Methods: We performed a candidate gene association study with replication, evaluating associations between polymorphisms in homologs of evolutionarily conserved clock genes and the timing of behavioral rhythms measured by actigraphy. For validated polymorphisms, we evaluated associations with transcript expression and time of death in additional cohorts. Results: rs7221412, a common polymorphism near period homolog 1 (PER1), was associated with the timing of activity rhythms in both the discovery and replication cohorts (joint p = 2.1 × 10-7). Mean activity timing was delayed by 67 minutes in rs7221412GG versus rs7221412AA homozygotes. rs7221412 also showed a suggestive time-dependent relationship with both cerebral cortex (p = 0.05) and CD14+CD16- monocyte (p = 0.02) PER1 expression and an interesting association with time of death (p = 0.015) in which rs7221412GG individuals had a mean time of death nearly 7 hours later than rs7221412AA/AG. Interpretation: A common polymorphism near PER1 is associated with the timing of human behavioral rhythms, and shows evidence of association with time of death. This may be mediated by differential PER1 expression. These results may facilitate individualized scheduling of shift work, medical treatments, or monitoring of vulnerable patient populations. ANN NEUROL 2012;72:324-334.