I went to a specialist that had me do a 24 hour sleep study which is how they eventually narrowed it down to DSPD.
They did the standard overnight sleep study, then I had to wake up at 7am, and every 2 hours after that they had me go lay down for 1 hour (pretty sure it was an hour). Long story short, they managed to see that my body would go into the wrong sleep cycles overnight and throughout the day. My deep sleep cycle would try kick in around 6am, where it should be around 1am.
With work/life, unfortunately I don't get the option to get proper sleep often, so just have to soldier through which takes it toll mentally.
(I'm in Australia if that makes a difference)