The object of this study was to understand why breast cancer frequently occurs in the left breast among both women and men.Moreover, recent results showed that the left side of the body also is more prone to melanoma than the right side.A literature search showed that current explanations for left-sided breast cancer include handiness, size difference, nursing preference, and brain structure.However, men are affected even more by left laterality than women, thus many of these explanations are unconvincing.Increasing rates of skin melanoma have been associated with immune-disruptive radiation from FM/TV transmitters.Geographical areas covered by several transmitters show higher incidences of melanoma than areas covered by one transmitter.Here we show that a high prevalence of breast cancer and melanoma on the left side of the body may be a logical consequence of sleeping in beds having mattresses containing wave-reflecting metal springs.We found that people tend to sleep for longer periods on their right side, apparently to avoid disturbance by the heartbeat.This puts the left side farther away from the field-attenuating influence of the metal springs in the mattress; thus the left side will spend, on average, more time exposed to stronger combined fields from incident and reflected waves.This hypothesis may also explain why body parts furthest away from the mattress (trunk and upper arms for men; lower limbs and hips for women) have higher melanoma rates than the sun-exposed face area.The implications of this study should promote a critical consideration of population exposure to electromagnetic fields, especially during the night.Wave-reflecting beds should be avoided as e.g.in Japan where melanoma only reaches 3% of the Swedish incidence.