This research study will evaluate changes in resting metabolic rate, appetite, and heart rate variability following overnight exposure (8 h/night) to normobaric hypoxia (NH) or normobaric normoxic (NN). In randomized order, participants will sleep one night in NH conditions (~15% oxygen; achieved with nitrogen dilution, equivalent to ~8500 feet elevation) and another night in NN (control) conditions (~20% oxygen; achieved with nitrogen dilution, equivalent to ~1000 feet elevation).

Weight loss is a common factor reported in lowlanders traveling to high altitudes. Although the exact cause is unknown, studies have shown that weight loss at high altitude is associated with the amount of time and severity of the altitude exposure. Upon arrival to high altitude (≥ 5000 feet), numerous physiological adaptations occur that may promote weight loss, such as increased energy expenditure and reduced appetite. The breadth of current altitude research is limited by the logistical and monetary constraints of traveling to remote locations or access to an altitude chamber. A more logistically feasible alternative is the simulation of low oxygen conditions at sea level, using a commercially available tent and generator system to reduce ambient oxygen concentrations. The current research study will evaluate changes in resting metabolic rate, appetite, and heart rate variability following overnight exposure (8 h/night) to normobaric hypoxia (NH) or normobaric normoxic (NN). In randomized order, participants will sleep one night in NH conditions (~15% oxygen; achieved with nitrogen dilution, equivalent to ~8500 feet elevation) and another night in NN (control) conditions (~20% oxygen; achieved with nitrogen dilution, equivalent to ~1000 feet elevation). In the morning, following NH or NN exposure, resting metabolic rate will be measured using indirect calorimetry, appetite using an ad libitum buffet meal protocol, and heart rate variability by electrocardiogram. It is hypothesized that exposure to overnight NH will result in increased resting metabolic rate, decreased appetite, and decreased heart rate variability.

Inclusion Criteria:

• Men and women aged 20 - 45 years
• Body mass index between 18.5 - 24.5 kg/m2
• Born at altitudes less than 2,100 meters (~7,000 feet)
• Currently residing in Tallahassee, Florida or the surrounding area
• Not taking any medication(s) that interfere with metabolism or oxygen delivery/transport for four weeks before and throughout the entire study (e.g., includes sedatives, sleeping aids, tranquilizers, and/or any medication that depresses ventilation, diuretics, alpha, and beta-blockers).
• Willing to refrain from smoking, vaping, chewing tobacco, and dietary supplement use throughout the entire study
• Willing to spend two overnight visits on the Florida State University campus.

Exclusion Criteria:

• Living in areas that are more than 1,200 m (~4,000 feet), or have traveled to areas that are more than 1,200 m for five days or more within the last two months
• Metabolic or cardiovascular abnormalities, gastrointestinal disorders, or any condition that interferes with metabolism or oxygen delivery/transport (e.g., kidney disease, diabetes, cardiovascular disease, etc.)
• Prior diagnosis of apnea or other sleeping disorders
• Prior diagnosis of high-altitude pulmonary edema or high-altitude cerebral edema
• Prior diagnosis of anemia or sickle cell anemia/trait
• Present condition of alcoholism, anabolic steroid, or other substance abuse issues
• Women who are pregnant, lactating, planning to become pregnant, or who have had an irregular menstrual cycle in the past six months.
• Allergies or intolerance to foods included in the standardized or ad libitum meal (e.g., lactose intolerance/milk allergy)