All subjects will be advised to stop using continuous positive airway pressure (CPAP) for at least 7 days as previous study showed that severity of obstructive sleep apnea (OSA) did not increase further after the first week of CPAP withdrawal. Subjects will then be invited to undergo a limited sleep study using "EMBLETTA" portable diagnostic system (MEDCARE, Iceland). The EMBLETTA is a pocket-sized digital recording device. It is a multi-channel transducer, providing an apnea-hypopnea index (AHI) based on recording time. It also detects both respiratory and abdominal efforts through the effort sensor and can differentiate between obstructive and central events. Built-in position sensors are also available to differentiate supine from non-supine events. Respiratory events are scored when desaturations of at least 4% occurred in the absence of moving artefacts and irrespective of co-existing changes in snoring or heart rate. The EMBLETTA operates on battery power, with the internal memory storage of 16 MB, which allows approximately 12 hours of data collection. The EMBLETTA default settings for apneas and hypopneas will be used in this study. An apnea is defined as a decrease in airflow by 80% of baseline for at least 10 seconds. The EMBLETTA default maximum apnea duration is set at 80 seconds. A hypopnea is defined as a decrease in airflow by 50% of baseline for at least 10 seconds. The EMBLETTA default maximum hypopnea duration is set at 100 seconds. The EMBLETTA AHI used for analysis is automatically analysed by the EMBLETTA software. The EMBLETTA device has been validated against hospital-based polysomnography in the Hong Kong Chinese population. The EMBLETTA is a highly sensitive and specific screening device in quantifying AHI and differentiating obstructing and supine events when compared against polysomnography (PSG) in patients with suspected OSA. Patients will be instructed how to operate the EMBLETTA device for the sleep recording and estimate their time of sleep. Those who have failed the EMBLETTA sleep study will be arranged to have a second home EMBLETTA sleep study. Patients who are symptomatic of OSA with a negative home EMBLETTA sleep study will be arranged to have a hospital-based PSG.

All subjects will undergo three-dimensional (3D) computed tomography (CT) scans of the head/neck region to evaluate the upper airway and craniofacial structures. Craniofacial restriction is determined by measuring the size of the maxillomandibular volume based on the mandibular cephalometric landmarks (left and right condylion, left and right gonion, anterior nasal spine (ANS) and menton) as per previous published study. The patient is positioned in the natural head position with a closed lip and bite. All measurements will be made by a single assessor and the analysis will be performed blind to knowledge of weight loss and AHI data.

All subjects will go through several measurements. These include assessment of subjective sleepiness with the Epworth Sleepiness scale (ESS), anthropometric measurements (body mass index, neck and waist circumference) and blood pressure.

The ESS is a questionnaire specific to symptoms of daytime sleepiness and the patients are asked to score the likelihood of falling asleep in eight different situations with different levels of stimulation, adding up to a total score of 0 to 24.