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Objectives: Sleep apnea is suggested to be associated with cancer risk, but results are heterogenous, and few studies are population-based. We aimed to assess risk associations between self-reported witnessed apnea during sleep and specific cancers in a population-based cohort.

Methods: This retrospective cohort study analyzed questions on witnessed sleep apnea in relation to incident cancer in the Northern Sweden Health and Disease Study. Cancer diagnoses were derived from the Swedish Cancer Registry and characterized as 12 different cancer types. Cox regression models adjusted for age, sex, ever smoking, body mass index, and education were used to assess risk associations.

Results: In total, 82,059 participants were included, and 10,668 (13 %) reported witnessed sleep apnea. They were followed for 9.0 (SD 4.7) years and 4030 incident cancers were diagnosed. Self-reported witnessed sleep apnea was independently associated with incident lung cancer with an adjusted hazard ratio (aHR), 1.78 (95 %CI 1.16, 2.73) p = 0.008 and breast cancer aHR, 1.39 (95 %CI 1.04, 1.84) p = 0.023. The risk for lung cancer was driven by an association with lung adenocarcinoma aHR, 2.16 (95 %CI 1.19, 3.91) p = 0.01. There was a multiplicative effect on ever smoking and reporting witnessed apnea for lung cancer with an aHR, 5.27 (95 %CI 3.07, 9.05) p < 0.001.

Conclusions: Self-reported witnessed sleep apnea is associated with an increased risk of developing lung- and breast cancer. There is a multiplicative effect of reporting witnessed sleep apnea and ever-smoking with an over 5 times increase on the hazard for lung cancer.