The Genetic Basis of Sleep and Sleep Disorders is a collection of papers on the subject, which has a chapter specifically on DSPD. Short story is, there are definitely genetic mutations which influence DSPD. In fact, DSPD seems to be influenced by a larger number of genes than it's cousin disorder, Advanced Sleep Phase Disorder.

I have DSPD, so I've been going to my local university library to study from this book.