Does an app based short intervention improve quality of sleep?

INTERVENTION: Method of randomisation: stratified block randomization (strata: gender, sleep quality (RIS total score <12 vs >=12), use of sleep medication). 1. Refresh: an app‐based short intervention to improve sleep quality 2. Waitlist control The intervention is self‐paced, but the recommended duration is 8 weeks (1 chapter per week). There are three assessments (baseline, post‐intervention (+8 weeks), follow‐up (+26 weeks). CONDITION: Poor sleep quality ; Nervous System Diseases ; Sleep disorders PRIMARY OUTCOME: Sleep quality is measured using the Regensburg Insomnia Scale (RIS) Total Score at baseline and post‐Intervention (8 weeks) SECONDARY OUTCOME: ; 1. Sleep quality is measured using the Regensburg Insomnia Scale Total Score at baseline and follow‐up (26 weeks); 2. Depression symptoms are measured using the depression scale from the Patient Health Questionnaire (PHQ‐9 ) Total Score at baseline, post intervention (8 weeks) and follow‐up (26 weeks); 3. Acceptance of the training is measured using the Working Alliance Scale adapted for Online Interventions at Post Intervention (8 weeks) and usage of the intervention (during intervention period); INCLUSION CRITERIA: 1. Over 18 years of age 2. Fluent in German 3. Access to the Internet during the intervention period