The objective of this study was to investigate the effect of age on the association between preferred timing of sleep and waking activities and a coding-region variable number tandem repeat (VNTR) polymorphism in the clock gene PER3. We have previously reported this polymorphism to associate with diurnal preference and delayed sleep phase syndrome (DSPS). Participants (n = 1590; 707 males and 883 females) completed the Horne-Ostberg (HO) questionnaire for diurnal preference and provided a DNA sample. Overall HO scores were plotted against age. The 5% extremes and intermediates were selected for genotyping. Frequencies of the PER3 4- and 5-repeat alleles were examined in separate age groups (18-29, 30-39, 40-49 and 50+ years of age). The 4-repeat allele was significantly more frequent in evening types, and the 5-repeat allele more frequent in morning types (Fisher's exact test, P = 0.016). Analysis in the four age groupings revealed that the strength of this association attenuated with age and was significant only in the youngest group (18-29 years). These results extend our previous finding of an association between the PER3 VNTR and diurnal preference. They also demonstrate that diurnal preference in young people is more closely associated with this polymorphism than it is in other age groups.