Increasing rates of skin melanoma have been associated with immune-disruptive radiation from FM/TV transmitters [O. Hallberg, A theory and model to explain the skin melanoma epidemic, Melanoma Res. 16 (2006) 115-118; O. Hallberg, A reduced repair efficiency can explain increasing melanoma rates, Eur. J. Cancer Prev. 17 (2008) 147-152; O. Hallberg, O. Johansson, Melanoma incidence and frequency modulation (FM) broadcasting, Arch. Environ. Health 57 (2002) 32-40; O. Hallberg, O. Johansson, FM broadcasting exposure time and malignant melanoma incidence, Electromagn. Biol. Med. 24 (2005) 1-8; O. Hallberg, Radio TV towers linked to increased risk of melanoma, Report, available at: http://foodconsumer.org/7777/8888/C_ancer_31/120907442007_Exclusive_report_Radio_TV_towers_linked_to_increased_risk_of_melanoma.shtml , 2007 (accessed 2007)]. 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 farthest 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.