Delayed sleep phase disorder (DSPD) is the most common circadian rhythm sleep disorder. Persons with DSPD have great difficulties falling asleep and waking up at conventional times. To diagnose DSPD this delayed sleep-wake rhythm should cause social impairment and distress for the individual. Evening melatonin and morning bright light are the recommended treatments. The overall aim of this thesis was to evaluate at-home treatment with Light therapy (LT) and the feasibility of adding cognitive behavior therapy (CBT) to LT in DSPD, furthermore prevalence, diagnostic aspects and associated factors were investigated. Study I included 673 randomly selected individuals aged 16–26 years. The prevalence of DSPD was 4.0%. Unemployment (defined as an absence of educational or work activities) and an elevated level of anxiety were associated with DSPD. In study II, dim light melatonin onset (DLMO) was measured in healthy adults. Time for DLMO DLMO (Mean±SD) was 20:58±55 minutes. Studies III, IV, and V present results from a randomized controlled trial examining the feasibility of CBT as an additive treatment to LT with scheduled rise times, in persons with DSPD. Sleep onset and sleep offset was significantly advanced from baseline (03:00±1:20; 10:22±2:02 respectively) to the end of LT (01:27±1:41; 08:05±1:29, p<0.001 respectively). This advancement was predicted by consistent daily usage of the LT-lamp. At the follow-ups after LT and CBT or LT alone, sleep onset remained stable, sleep offset was delayed, and sleep difficulties were further improved, but there was no significant group interaction over time. There was a significant group interaction over time in the severity of anxiety and depressive symptoms, both in favor of the LT+CBT group. Conclusively, DSPD was common among adolescents and young adults and it was associated with unemployment and elevated levels of anxiety. DLMO appeared in the expected time range in healthy working adults. At-home treatment with LT with scheduled rise times advanced sleep-wake rhythm and improved sleep difficulties in DSPD. Even though sleep-wake rhythm was not further advanced or better preserved in the participants that received LT+CBT compared to LT alone, the addition of CBT to the treatment regimen was feasible and well accepted.