The purpose of this research study is to see if obstructive sleep apnea (OSA) is associated with abnormalities in fat metabolism. Through this research study, the Investigator will evaluate how fat is metabolized in people with and without sleep apnea, what substances the fat tissue releases, and how these substances might change the way the body uses energy and sugar.

Aim 1 Group:

Inclusion criteria:

• Age between 18-70 years
• Ability to provide consent

Exclusion criteria

• Body Mass Index (BMI) > 40 kg/m2
• Prevalent myocardial infarction, coronary revascularization, heart failure, and stroke
• Type 1 or Type 2 diabetes mellitus
• Current or prior use of PAP or oral appliance therapy for OSA
• Use of oral corticosteroids
• Unstable medical conditions that may preclude enrollment in the protocol such as: uncontrolled angina and/or congestive heart failure, uncontrolled blood pressure or resistant hypertension, severe chronic obstructive pulmonary disease, cancer, and active psychiatric disease (e.g., major depression)
• Other sleep disorders (e.g., circadian rhythm disorder, self-reported habitual sleep duration < 6h)
• Use of supplemental oxygen during wakefulness or sleep
• Central sleep apnea, obesity hypoventilation syndrome or Cheyne-Stokes respiration based on the home sleep test
• Resting awake Oxygen Saturation (SpO2) < 90%

Aim 2 Group:

Inclusion criteria:

• Age between 18-70 years
• Ability to provide consent
• Moderate-to-severe OSA (AHI ≥ 15/h);
• Demonstration of Positive Airway Pressure (PAP) use of > 4 h/night on 70% of the nights during 1-week run-in period
• Successful completion of all of the procedures for Aim 1.

Exclusion criteria:

• BMI: ≥ 40 kg/m2
• Type 1 or Type 2 diabetes mellitus
• Current use of PAP or oral appliance therapy for OSA
• Commercial driver or report of motor vehicle accident or near-miss due to sleepiness within the 2 previous years
• Epworth sleepiness score of 18 or more
• Use of oral corticosteroids
• Participation in another clinical trial
• Unstable medical conditions that may preclude enrollment in the protocol such as: uncontrolled angina and/or congestive heart failure, uncontrolled blood pressure or resistant hypertension, severe chronic obstructive pulmonary disease, cancer, and active psychiatric disease (e.g., major depression)
• Other sleep disorders (e.g., circadian rhythm disorder, self-reported habitual sleep duration < 6h)
• Use of supplemental oxygen during wakefulness or sleep
• Central sleep apnea, obesity hypoventilation syndrome or Cheyne-Stokes respiration based on the home sleep test
• Resting awake SpO2 < 90%