The purpose of this study is to help us understand how HIV and/or Anti-Retroviral Therapy (ART) may predispose individuals to Obstructive Sleep Apnea (OSA). The traditional risk factors for OSA are weight and age. However, people living with HIV on ART seem to have OSA even when they are thin and young. The study involves a detailed physiological sleep study and an MRI of the head and neck to understand the underlying cause of OSA in those with and without HIV.

Participants who have sleep apnea and are using positive airway pressure (PAP) with and without HIV will be enrolled if they otherwise meet all the inclusion criteria, and none of the exclusion criteria.

Subjects will undergo two overnight sleep studies (polysomnograms, PSGs) and a MRI scan of the upper airway. The first sleep study will evaluate the severity of sleep disordered breathing and will be identical to a clinical PSG. The second sleep study will measure the physiological traits most important for OSA, such as upper airway anatomy, control of breathing, sleep stability, and upper airway muscle activation. These measurements are performed while subjects sleep using PAP, but the PAP level is adjusted during the night, and the subject's response is measured. The MRI scan will be of the head and neck and will occur during the evening time. Subjects will be encouraged to sleep during this portion of the study, as well.

Inclusion Criteria:

• Men and women with physician-diagnosed OSA who have been prescribed CPAP Ages >= 40 years old
• BMI 20 - 35 kg/m2
• For HIV group: physician diagnosis of HIV and viral suppression

Exclusion Criteria:

• Not compliant with CPAP (by Medicare criteria, e.g. ≥4 hours per night on 70% of nights during a consecutive 30-day period via direct download or visual inspection of usage data).
• Use of medication that affects the traits, e.g. narcotics or sedative-hypnotics
• Any cardiovascular, pulmonary or renal disease other than well controlled hypertension or asthma.
• Pregnancy
• Currently smoking
• Any respiratory disorder other than OSA or well controlled asthma
• contraindication to MRI

Men and women with physician-diagnosed OSA who have been prescribed and are using positive airway pressure (e.g. CPAP). Also, men and women with physician-diagnosed HIV who achieved viral suppresion.