I had the same problem.
I found out I have sleep apnea. I have a smaller than average throat for my body size. I did a sleep study and found I was getting only about 60% of the air I needed, and occasionally stopped breathing. Once I started using a CPAP, I don't have to sleep 2-3 hours when I get home from work.
I'd check with your primary care doc and see what they recommend. A sleep study is really the only way to tell what actually goes on when you sleep for brain activity and breathing.
On a side note, while still at the hospital and recovering from my knee arthroscopy, as I was waking up, I stopped breathing several times. Nothing serious, but as I was partially awake, the nurse kept on telling me to breath, and I'd go, oh, I should breath, take a breath, and then not take another one until she told me to again. Once the fog of anesthesia went away, I didn't have this problem. Fun Stuff.