We classified 64 patients with chronic delayed sleep phase syndrome (DSPS) into 
the primary (n = 53) and secondary (n = 11) group according to presence or 
absence of such signs as difficulty in waking up which appeared much earlier 
than the onset of DSPS. The age at the onset of the early signs concentrated in 
adolescence. The familial occurrence of DSPS was noted in 11 patients of the 
primary group. In human leukocyte antigen (HLA) typing, the incidence of DR1 
positivity alone was significantly higher in DSPS patients than in healthy 
subjects. Minnesota Multiphasic Personality Inventory revealed high scores on 
depression, psychoasthenia and hypochondriasis. We suggest that a predisposition 
to DSPS includes biological, genetic, social and psychological factors, various 
combinations of which may lead to DSPS.