I think it originates as a mood disorder, which is worsened by a sleep disorder. My husband is worse after especially long days at work. Comes home, overly mushy at first, then rambling, irritable, argumentative, repeats himself a lot, obsessed on one topic, slurred speech, cries easily, then falls asleep right away. It's like a 180 pound cranky toddler. My husband also has high blood pressure and sleep apnea, which is treated with a CPAP. His mood is more normal when he uses his CPAP all night and can work from home, skipping the long commute.