Sounds like you may have sleep apnea. I always felt extremely tired during the day even having slept for 8 hours, yawning all the time. Went to the doctor and he referred me to a sleep specialist. They did a sleep study and i was diagnosed with sleep apnea. You may have it and not even know but you should get it checked out.
Sleep apnea Symptoms - Mayo Clinic
The way it was described to me is basically, you stop breathing during the night or can't catch your breathe for a very short amount of time. In that time you wake up and go back to sleep 30+ times a night so you never have a full nights sleep and all those times you wake up you won't remember it in the morning.
I have to sleep with a CPAP machine now, I haven't been able to faithfully sleep with it every night so it's hard for me to tell if anything has changed but i'm still tired during the day. You should get yourself checked because sleep apnea is very dangerous and can cause heart disease.