The timing of clock gene expression in human leukocytes was investigated following a phase-advancing light stimulus to determine whether the response is wavelength- and/or age-dependent. PERIOD3 (PER3) clock gene expression in leukocytes and plasma melatonin were analyzed before and after monochromatic blue and green light exposure. Significant phase advances were observed in the peak timing of both PER3 expression and melatonin following blue but not green light. The amplitude of the PER3 rhythm at baseline was significantly reduced with age. However, age did not affect the response of the PER3 rhythm to light.