OBJECTIVE: Shift work encompasses a broad range of work time arrangements. However, how shift work affects the circadian expression of clock genes remains to be explored. The objective of this study was to evaluate the pattern of clock gene expression in shift workers in the field. METHODS: We examined clock gene expression in Japanese men who work: (1) one night shift followed by a day off (caregivers: nurses and doctors; the one-night group); (2) three or more consecutive night shifts (factory workers; the consecutive-night group); or (3) daytime only (the daytime group), using beard follicle samples. The expression of Period3, Nuclear Receptor Subfamily 1 Group D Member 1 (Nr1d1), and Nuclear Receptor Subfamily 1 Group D Member 2 (Nr1d2) was examined by real-time polymerase chain reaction. RESULTS: Period3 expression in the daytime and one-night groups together with Nr1d2 expression in the one-night group fitted a 24-h-period cosine curve better than in the consecutive-night group (p = 0.004, 0.012, and 0.001, respectively). The level of overall Period3 gene expression, calibrated with that of 18S-rRNA, was decreased in the consecutive-night group compared with that in the daytime group (p = 0.006). The patterns of Period3 and Nr1d2 expression in the daytime and one-night groups were more coherent than those in the consecutive-night group. CONCLUSIONS: These results suggest that night shift work affects the rhythms and levels of circadian Period3 and Nr1d2 expression dependent on the shift schedule or type of the shift; however, there is substantial variation between individuals.