There is also a reverse causality that's often overlooked - People getting obese because they develop sleep apnea. Happened to a relative. Developed allergy-related sleep apnea resulting in significantly reduced sleep quality and quality of life. Was often awake until after midnight due to trouble falling asleep from breathing issues that occured when nose and throat would start to relax. After about two years of this, he let himself go and gain 15kg over the coarse of a single year.