Sleep hormone melatonin tested as painkiller for chronic back pain

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: To be eligible for the trial patients must fulfill all the following inclusion criteria: * Age 18 to 64 years * Understand and write Danish * Back pain for 3 months or longer * Back pain must be present on 'most days' or 'every day' within the past 3 months (will be checked by the question: 'In the past 3 months, how often did you have back pain? - response options: 'never'; 'some days'; 'most days'; 'every day') * Back pain must limit life or work activities on 'some days', 'most days', or 'every day' within the past 3 months. (will be checked by the question: 'In the past 3 months, how often did your back pain limit your life or work activities? - response options: 'never'; 'some days'; 'most days'; 'every day') * Average pain intensity of 4 or higher on 0-10 Numeric Rating Scale \[NRS\] in the past 7 days (ranging from 'no pain' to 'worst imaginable pain'). * All fertile women must use safe contraception (Spiral, birth control pills, contraceptive patch, contraceptive vaginal ring or gestagen injections) for 3 weeks before and 1 week after the trial. If the participants' normal lifestyle includes sexual abstinence, they do not have to use contraception. Instead, they can give an oral informed consent, that they will be sexually abstinent during the trial. A woman is considered non-fertile if she is sterilized, hysterectomized, bilateral oophorectomized or is postmenopausal. A woman is considered postmenopausal when vaginal bleeding has been absent for 1 year (reported by the participant). Exclusion Criteria: Patients will be excluded based on any of the following exclusion criteria: * Known abuse of alcohol or other substances * Self-selected non-user of e-boks * Opioid use (reported by participant) * Known malignancies within past 6 months (reported by participant) * Known fractures within past 4 months (reported by participant) * Known lumbar radiculopathy (reported by participant) * Known spinal stenosis (reported by participant) * Severe psychiatric disorders and/or psychotic symptoms evaluated by the investigator (reported by participant) * Suicide and self-damage thoughts (reported by participant) * Inflammatory/autoimmune arthritis (reported by participant) Patients with contraindications to Melatonin according to the Danish Medicines Agencys approved product information: * Moderate to severe kidney insufficiency (GFR \< 30 mL/min) * Moderate to severe liver insufficiency (ALAT must not be elevated more than 3-fold over highest reference level) * Auto-immune diseases * Epilepsy * Warfarin use * Benzodiazipin use (including hypnotics) * Fluvoxamin use (Ciprofloxacin, Norfloxacin) * Calcium antagonist use (Verapamil, Nifedepin) * Pregnancy or pregnancy-wish or breastfeeding (a negative pregnancy test has to be available for all fertile female patients at baseline) * Intolerance to melatonin For the EEG subgroup: If the anatomy of the outer ear making it impossible to do ear EEG monitoring If there have a perforation of the tympanic membrane (eardrum) If they have an ear tube in the tympanic membrane If their ear piercings that are not compatible with ear EEG. If they use anticoagulants