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 diffe
rent light intensities (20, 40, 80 and 3200 Ix) of both polarized and nonpo
larized light, as well as to a control, dark exposure. Each evening experim
ent 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 expos
ures. Blood samples were drawn at the start and end of each light-exposure
period and later assayed for melatonin by radioimmunoassay. When compared t
o control exposures, both polarized and nonpolarized light elicited signifi
cant suppression of plasma melatonin at each illuminance (P < 0.03 to P < 0
.0001), There were no significant differences between the effects of polari
zed 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 res
ponsive 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 regulat
ion.