Not OP but I was diagnosed by being sent to a sleep specialist. I complained of fatigue to my PCP so they checked me for sleep apnea and during the initial meeting I described my schedule and what is typical for me. At the end of the questioning I said "yeah, I've suspected I have a sleep disorder or something" and she said "you definitely have delayed sleep phase disorder" and BOOM it was in my chart.

Easiest diagnosis ever. It helps if you keep a sleep log for a little bit. I can't guarantee the ease of your diagnosis but mine was just being honest instead of lying and pretending to have a normal day walker schedule.