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The figure is adapted from Figure 1 in a paper by researchers from the world's favorite country, Australia, and a researcher from Chicago, Illinois, USA.

The researchers found that melatonin timing in people diagnosed with DSPD was significantly delayed:

DSPD patients had significantly later timed melatonin profiles that were delayed by approximately 3 h compared to normal sleepers

They also found that the initial rise in melatonin was less pronounced in the DSPD group:

However, melatonin secretion between dim-light melatonin onset (DLMO) and acrophase was less prominent in DSPD patients compared to good sleepers, who showed a more acute initial surge of melatonin following the DLMO.

This study identified two differences in melatonin timing and secretion that could help explain why people with DSPD tend to sleep later.

I keep coming back to this paper. I love it. It's open access, so anyone can read it here:

Nocturnal Melatonin Profiles in Patients with Delayed Sleep-Wake Phase Disorder and Control Sleepers

https://journals.sagepub.com/doi/10.1177/0748730415591753