Study aim: This study aims to use a new brainwave monitoring device that meassure brainwaves 24 hours a day to create better understanding of sleep-wake interplay as a biomarker for excessive daytime sleepiness because of obstructive sleep apnea. Study Method, Design, and Procedures Patients with sleep apnea referred to the Sleep Clinic at the Department of Ear, Nose, Throat, and Jaw Surgery in Køge for CPAP treatment, who also experience severe daytime sleepiness, will be offered participation in this study. The goal is for 15 participants to complete the entire protocol. Procedure Overview Initial Sleep Assessment: The study begins with an in-depth sleep assessment through polysomnography (PSG) followed by a Multiple Sleep Latency Test (MSLT). PSG: This sleep study evaluates sleep structure, confirms sleep apnea, and rules out other sleep disorders. MSLT: This test measures the brain's tendency to fall asleep during the day. Additional Home-Based Sleep Studies: Participants will undergo two more PSG studies in their homes. EEG Monitoring: After the initial assessments, participants will have a small EEG electrode implanted to monitor brain activity. The implant, a CE-certified device called 24/7 EEG™ SubQ, will be inserted under local anesthesia by an experienced surgeon. Once post-surgery swelling subsides (after about 7-10 days), continuous EEG monitoring will begin, where participants connect an external data storage device during the day. The device records for 12 weeks, and CPAP treatment begins around the 6-week mark. After 12 weeks, the EEG device is removed, and the study ends. Aditional Data Collection (Optional) During the 12 weeks of EEG recording, participants are asked to report: Sleep Diary: Log daytime sleep episodes, including planned naps and unintended naps. Questionnaires and Tests: Complete daily questionnaires about fatigue symptoms and perform a brief attention and reaction test. These tasks take 3-10 minutes each day and are accessible via smartphone or tablet. Interviews and Movement Tracking: Participants may also be invited to interviews on how sleepiness affects their daily life and asked to wear a motion-tracking watch to monitor activity.