The aim of this study was to compare the effects of polarized light versus nonpolarized light on melatonin secretion in healthy, humans (mean age, 25 years; N = 6). On separate evenings, each subject was exposed to four different light intensities (20, 40, 80 and 3200 lx) of both polarized and nonpolarized light, as well as to a control, dark exposure. Each evening experiment consisted of a 120 min dark exposure (0000–0200 h) followed by a 90 min light exposure (0200–0330 h). Subjects' pupils were dilated prior to exposures. Blood samples were drawn at the start and end of each light-exposure period and later assayed for melatonin by radioimmunoassay. When compared to control exposures, both polarized and nonpolarized light elicited significant suppression of plasma melatonin at each illuminance (P < 0.03 to P < 0.0001), There were no significant differences between the effects of polarized light and nonpolarized light at any illuminance. The two light stimuli modalities demonstrated very similar fluence–response relationships between illuminance and melatonin suppression. Thus, the human pineal gland is responsive to ocular exposure with polarized light in a dose-dependent manner similar to that of nonpolarized light, although no significant differences were detected between polarized and nonpolarized light on melatonin regulation.