Multi-night exposure to 60 Hz magnetic fields: effects on melatonin and its enzymatic metabolite.

Magnetic field-induced suppression of nocturnal melatonin in humans has been reported in occupational and residential studies, but not in laboratory-based exposure studies. The present study examined whether this contrasting pattern of results might be related to associated differences in exposure duration or to field-induced measurement instability over time. Thirty healthy young men were evaluated using a randomized, double-blind test protocol. Statistical analysis indicated that 4 consecutive nights of exposure to power-frequency magnetic fields at occupational intensity (resultant flux density = 28.3 microtesla, muT, [283 milligauss, mG]) had no differential effect on concentrations of melatonin or its major enzymatic metabolite (6-hydroxymelatonin sulfate, 6-OHMS) in daily morning urine samples, compared to equivalent no-exposure sham control conditions. The consistency of intra-individual urinary measurements over the 4 test nights also was quite high (P < 0.01) in the sham control condition. In contrast, repeated nightly exposure to the magnetic field was associated with reduced consistency. Morning urinary measures obtained after exposure on night 4 differed (P < 0.01) from similar measures obtained after the second and third exposure night. Thus, while the overall results of this study do not support the melatonin hypothesis, there is some suggestion of a possible cumulative effect of magnetic field exposure on the stability of individual melatonin measurements over time. Additional research with longer periods of controlled exposure may be warranted.