The evidence of magnetic field (MF) effects on melatonin production in humans is limited and inconsistent. Part of the inconsistencies might be explained by findings suggesting interaction with light in pineal responses to MFs. To test this hypothesis, we reanalyzed data from a previously published study on 6-hydroxy melatonin sulfate (6-OHMS) excretion in women occupationally exposed to extremely low-frequency MFs. Based on questionnaire data on exposure to light-at-night (LAN), and measurement-based MF data, the 60 women were classified to four groups: no MF, no LAN; MF, no LAN; no MF, LAN; MF, LAN. The lowest excretion of 6-OHMS was observed in the group of women who were exposed to both MF and LAN, and the differences between the four groups were significant (P < .0001). The result is based on low numbers, but supports the hypothesis that daytime occupational exposure to MF enhances the effects of nighttime light exposure on melatonin production.