Sleep apnea can cause that, might be something to take up with your doc.
I had to take my mom twice for a sleep test, she was prescribed a CPAP machine, even had a changed over to nose "pillows" (worn intranasaly) for better sleeping comfort but wimped out using, didn't use it long enough and still continues to be fatigued.
I have a brother-in-law who uses it faithfully, but then again has a higher severity of sleep apnea, and has done well, claims he never felt better.
It's always best to rule out any underlying physical causes, it isn't always the things mind related that are culprits.
I hope all works out for you, changing patterns is always difficult in the begining, I'm still working on developing better habits, like when I feel tired, go to bed, not fall asleep on the couch and then wake up after the late night news and go up to bed expecting to readily fall asleep.
Oh that habit of mine is horrible, broken sleep cycle can become a real nightmare(no pun intended) which we have to take seriously and look into the individual causes we often suffer from.
Keep us updated on any new findings.