Recent epidemiological studies have reported an increased risk of leukemia in adults and children near overhead high voltage powerlines at distances beyond the measured range of the direct electric and magnetic fields. Corona ions are emitted by powerlines, forming a plume that is carried away from the line by the wind. The plume generates highly variable disturbances in the atmospheric electric field of tens to a few hundred V/m on time scales from seconds to minutes. Such disturbances can be seen up to several hundred meters from powerlines. It is hypothesized that these random disturbances result in the disruption of nocturnal melatonin synthesis and related circadian rhythms, in turn leading to increased risk of a number of adverse health effects including leukemia. In support of the hypothesis, it is noted that melatonin is highly protective of oxidative damage to the human hemopoietic system. A review of electric field studies provides evidence that (i) diurnal variation in the natural atmospheric electric field may itself act as a weak Zeitgeber; (ii) melatonin disruption by electric fields occurs in rats; (iii) in humans, disturbances in circadian rhythms have been observed with artificial fields as low at 2.5 V/m. Specific suggestions are made to test the aspects of the hypothesis.