Recent studies have demonstrated a close relationship between circadian clock function and the development of obesity and various age-related diseases. In this study, we investigated whether messenger RNA (mRNA) levels of clock genes are associated with age, body mass index, blood pressures, fasting plasma glucose, or shift work. Peripheral blood cells were obtained from 70 healthy women, including 25 shift workers, at approximately 9:00 AM. Transcript levels of clock genes (CLOCK, BMAL1, PER1, and PER3) were determined by real-time quantitative polymerase chain reaction. Stepwise multiple regression analysis demonstrated that BMAL1 mRNA levels were correlated only with age (β = −.50, p < .001). In contrast, PER3 levels were correlated with fasting plasma glucose (β = −.29, p < .05) and shift work (β = .31, p < .05). These results suggest that increased age, glucose intolerance, and irregular hours independently affect the intracellular clock in humans.