Introduction 16.8% of the Danish adult population are obese (Body Mass Index\> 30 kg / m2). Obesity increases the risk of lifestyle diseases such as type-2 diabetes and non-alcoholic fatty liver. People with mental illness have an increased risk of developing obesity. Both obesity and certain mental disorders (bipolar disorder and schizophrenia) are associated with circadian rhythm disorders. Clinically, this may manifest as reduced sleep quality, depressive symptoms and increased fatigue, but also deregulation of a wide range of bodily processes subject to the circadian rhythm. In circadian rhythm disorders, the pattern of how mRNA of specific 'clock genes' is expressed in the cell may be affected. These clock genes are associated with obesity, bipolar disorder and schizophrenia. Despite the clear indications of an interplay between mental illness, obesity and circadian rhythm disorders, the relationship between these illnesses are largely unexplored. Aim The aim of this study is to investigate circadian disturbances in people with and without obesity, as well as people with obesity and a comorbid diagnosis of either schizophrenia or bipolar disorder. Methods The study population will consist of: 1. People with obesity and schizophrenia (N=22) 2. People with obesity and bipolar disorder (N=22) 3. People with obesity without psychiatric disease (N=22) 4. People with BMI 18.5 - 25kg/m2 and no psychiatric disease (N=20) Study Procedure Participants will visit the clinic 2 times. At each visit participants fill in questionnaires and perform physical tests. Between visit 1 and 2, participants will over a 2-day period (at-home), collect biological samples (Four hair- and six saliva samples per day). In addition, participants will wear accelerometers and continuous glucose monitors (CGMs) for a total of 8 days, including the 2-day sampling period. Sampled hair follicles are analyzed for relative expression of clock gene mRNA. Saliva is analyzed for cortisol- and melatonin content. The four participants groups are analyzed and compared on daytime variation in mRNA expression, cortisol- and melatonin concentration, and body temperature. Perspectives A comparison of patient groups presenting with mental disease, obesity and circadian disturbances may provide new insight into the association between these diseases.