Complex sentence: A 2012 study has shown that hypoxia (an inadequate supply of oxygen) that characterizes sleep apnea promotes angiogenesis which increase vascular and tumor growth, which in turn results in a higher incidence of cancer mortality.
Split into simpler sentences: A 2012 study has shown that hypoxia (an inadequate supply of oxygen) that characterizes sleep apnea promotes angiogenesis which increase vascular and tumor growth. Which in turn results in a higher incidence of cancer mortality.