This article confirms something I have been seeing more and more in my clinic - people who struggle with obesity are feeling excessively sleepy, and it's not due to disrupted sleep or obstructive sleep apnea. Since OSA does cause sleepiness, and is strongly associated with obesity, it's difficult for me to tell which obese people are sleepy from OSA versus from obesity without a sleep study.
The article also discusses research that shows that people who are depressed can be excessively sleepy from the depression itself, rather than from a separate sleep disorder like OSA. I see this too in my sleep clinic, but less often, and it's still important to rule out sleep disorders like OSA.
Bottom line: If you're feeling sleepy in the daytime, despite getting enough sleep, talk to your doctor or a sleep physician.