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.

Problématique : Certaines personnes souffrent d'avoir un horaire de sommeil trop tardif qu'elles n'arrivent pas à modifier pour satisfaire les exigences liées à leur emploi ou à leurs études. Ces individus souffrent de privation de sommeil et de somnolence lorsqu'ils doivent se conformer à un horaire socialement acceptable. Malgré sa prévalence importante, l’étiologie d’un horaire trop tardif reste méconnue. Objectif: Évaluer des facteurs pouvant contribuer au maintien d’un horaire de sommeil tardif chez des jeunes adultes. Méthodes : Quatorze jeunes adultes se plaignant d’un horaire de sommeil trop tardif ont été comparés à des sujets appariés qui avaient un horaire de sommeil adapté. L’heure de coucher (HC) était après minuit pour les sujets tardifs et avant minuit pour les sujets adaptés. Les sujets étaient admis au laboratoire 5h avant l’HC et garder en obscurité pour 6h. Ils étaient ensuite exposés à 1,5h de lumière bleue. La mélatonine salivaire et la vigilance subjective étaient mesurées aux 30 minutes. La suppression de mélatonine a été utilisée pour déterminer la sensibilité circadienne à la lumière. Le dim light melatonin onset (DLMO) a été utilisé pour déterminer la phase circadienne. Résultats : Le DLMO survenait plus tard dans le groupe tardif que dans le groupe adapté. Il n’y avait pas de différence de suppression de mélatonine après 1,5h d’exposition à la lumière. Néanmoins, une corrélation entre la sensibilité à la lumière et la phase circadienne a été trouvée dans le groupe tardif. Les sujets tardifs présentaient aussi une augmentation plus lente de la somnolence subjective en soirée. Conclusion : Nos résultats suggèrent qu’une phase circadienne en délai, une augmentation plus lente du besoin de dormir et une sensibilité circadienne à la lumière accrue contribuent à la plainte d’un horaire de sommeil trop tardif. / Problem: Some people suffer from having a delayed sleep schedule that they can’t modify to satisfy school/work requirements. These individuals suffer from sleep deprivation and sleepiness when they have to comply with a socially acceptable schedule. Despite its high prevalence, the etiology of a delayed schedule remains unknown. Objective: This study aims to elucidate factors that might contribute to the maintenance of a delayed sleep schedule in young adults. Methods: Fourteen young adults (8 women) complaining of delayed sleep schedule were compared to matched subjects with an adapted sleep schedule. Habitual bedtime (HB) was after midnight in all delayed subjects and before midnight in all adapted subjects. Subjects were admitted 5h before HB and kept in dim light for 6h. They were then exposed for 1.5h to blue light. Salivary melatonin and subjective sleepiness were assessed every 30 min. Melatonin suppression was used to measure circadian sensitivity to light. Dim light melatonin onset (DLMO) was used to estimate circadian phase. Results: DLMO was later in the delayed than in the adapted group. There was no difference for melatonin suppression over the 1.5h of light exposure. However, in the delayed group, there was a significant correlation between DLMO and melatonin suppression. There was a smaller increase of subjective sleepiness in the delayed subjects than in the adapted subjects before HB. Conclusions: Our results suggest that a delayed circadian phase, a slower build-up of sleep propensity and an enhanced circadian sensitivity to evening light contribute to the complaint of a delayed sleep schedule.