What Causes Sleep Apnoea to Vary Between Nights? A Multi-Night Detailed Study

Participants with high standard deviation (>66%) in apnoea hypopnoea index (AHI) based on 30 nights of at-home assessment prior to the study. Consent + Baseline Data [~2 hours]: Once the participant signs consents, Each participant will first be assessed for their height, weight, facial dimensions (Interzygomatic distance, inter gonion distance, ala-tragus distance, and bilateral gonion to menton distance), mandibular range of motion, neck length and circumference, and Mallampati score. They will then be asked to complete a Insomnia Severity Index (ISI), Pittsburgh Sleep Quality Index, Epworth Sleepiness Scale, Temporomandibular Joint Pain Screener, Research Diagnostic Criteria questionnaire for TMD, and Steigerwald and Maher TMJ Disability Index. Standard Diagnostic Sleep Study (48 participants, nights 1 to 3, ~8h per night) During these nights, the following effect modifiers will be assessed: • Polysomnography (PSG): to assess sleep stages, AHI, arousal thresholds, sleep hypoxemia, and muscle function (endotypes). • Body position: using under-mattress sleep analyser. • Neck extension and flexion: assessed using an accelerometer motion tracking system and real-time camera-based monitoring continuously recording audio-visual data of head, neck, body, and jaw positions. • Jaw position: assessed by both accelerometer/magnetometer tracking and real-time camera-based monitoring of jaw movement during sleep. • Jaw muscle activity: involuntary jaw activity will be assessed using surface EMG of the masseter muscles. Extended Respiratory Physiology Study (~24 participants, nights 4 to 6, ~8h per night) Participants will be fitted with a nasal mask with pneumotachograph, pressure-tipped catheters (epiglottic and choanal), and mask pressure transducer, but this time participants will be asked to sleep with this equipment to upper-airway physiology during sleep. • Sleep collapsibility analyses: After topical decongestant and anaesthetic spray, a nasal catheter will be inserted to measure epiglottic pressure. A nasal mask fitted with a calibrated airflow sensor will be used. Mouth will be taped to ensure nasal breathing and can be easily replaced as required. Epiglottic pressures will be analysed to assess breathing effort, work of breathing, and distinctions between effective and ineffective snoring, using established analytical methods. Inspiratory and expiratory flow–time curves will be evaluated to detect the site of collapse (epiglottic, retrolingual, or palatal) for each breath exhibiting flow limitation. CPAP Manipulation (Drop) Protocol (Night 7 only, ~8 hours): In addition to all the tasks performed in Nights 4 to 6, o Participants will undergo active and passive Pcrit assessments during staged gradual CPAP reductions (10 cmH2O to 2 cmH2O). o The first few breaths after CPAP drop will be analysed for anatomical stability. o The following breaths will be analysed for neuromuscular compensation.