PURPOSE OF REVIEW: For over 30 years, delayed sleep phase disorder (DSPD) has been defined as a debilitating sleep condition. Recently, there is more awareness of DSPD in young people, yet considerable information is needed to understand its cause and treatment. This review describes the latest research findings describing the clinical features, cause, and treatment of DSPD. RECENT FINDINGS: The prevalence of DSPD in adolescents and young adults ranges from 1 to 16%. The impact on the individuals is significant, particularly in the domains of school/work performance and mental health. We describe various contributing factors including reduced homeostatic sleep pressure, a lengthened and delayed circadian rhythm, insensitivity to clock-resetting morning light, and heightened cognitive activity. Evening melatonin administration as a sole treatment appears promising, as is a combination of cognitive-behavior therapy and morning bright light. SUMMARY: Recent findings suggest clinicians to be aware of the clinical features (i.e., significant daytime sleepiness, anxiety and depression symptoms, potential for school dropout) of DSPD, as several biological features underpinning this disorder are unseen in clinical settings. We advise clinicians to become familiar with exogenous evening melatonin administration, and cognitive and behavioral techniques to simultaneously treat the delayed circadian rhythm and associated sleep-onset insomnia.