Not sure if any of you are aware of this or suffer from it, but for well over a decade, actually since right around the time my POIS started in college, my sleep schedule became delayed (normal wake time is noon or later, and sleep time is 4 AM or later) and no matter what i do i cannot change it. Even when i worked 9-5 jobs i felt like crap during the day and slept in on weekends to catch up. DSPD causes a lot of feelings similar to "jet lag", fatigue, lack of energy, depression, anxiety, headaches, irritability, etc. I still never feel like i get deep, restful sleep even though i now have a CPAP machine and the apnea numbers are generally within a healthy range nowadays.
Long story short, after finding out that Delayed Sleep Phase is its own "disease" so to speak (actually more like a disorder of the Circadian Rythym or a Syndrome of various resulting symptoms) i finally found a good doctor who knows what it is and is helping me to treat it with melatonin, light therapy, and gradual schedule adjustment.
My current treatment is still early days. I may need stimulant medication to help wake up earlier. But i feel that it may ultimately help to further alleviate POIS symptoms