Mammalian circadian rhythmicity has recently been shown to be regulated at the genetic level by transcription-translation feed-back loops. Key molecular components such as Clock, Bmal-1, Timeless and three Period proteins have been isolated in mammals. In this study, we hypothesized that polymorphisms at the level of one of these genes - HPER1 - could be associated with differential morningness-eveningness tendencies. The sample comprised 463 middle-aged participants enrolled in the Wisconsin Sleep Cohort Study. Diurnal preferences were evaluated using the Horne-Ostberg questionnaire. An A to G synonymous substitution at position 2548 was identified in HPER1 c-DNA sequence by comparing available sequence data. This polymorphism was verified by sequencing and typed using established oligotyping techniques in all subjects, yielding allele frequencies of 0.85 and 0.15 for HPER1 2548G and HPER1 2548A, respectively. Morningness-eveningness scores were then compared between genotype groups. In contrast to data previously published using a Clock polymorphism, scores did not differ significantly across HPER1 groups. These results suggest that polymorphism at the level of HPER1 does not significantly modulate morningness-eveningness tendencies in the general population.