This study plans to learn more about metabolic responses to aerobic exercise at different times of the day (morning or evening) under fasting versus fed conditions.

This study plans to learn more about metabolic responses to aerobic exercise at different times of the day (morning or evening) under fasting versus fed conditions. Specifically, the investigators will explore the effect of fasted and fed AM and PM aerobic exercise on exercise energy expenditure and substrate oxidation, explore the effect of fasted and fed AM and PM aerobic exercise on plasma metabolites and lipids, and explore the effect of fasted-AM and Fasted-PM on 24h energy expenditure, 24h substrate metabolism, 24h glucose, and 24h profiles of hormonal regulators of energy metabolism

Inclusion Criteria:

• Age 18-40 years
• Body Mass Index 18.5-<40 kg/m2
• Recreationally active: defined as >100 minutes per week of voluntary exercise at moderate intensity or greater and >60 min per day of total habitual physical activity (i.e., work related, transportation related) at moderate intensity or greater, over the past 3 months.
• No self-report of acute or chronic disease (cardiovascular disease (CVD), diabetes, pulmonary, gastrointestinal disorders, and orthopedic problems in particular).
• No plans to relocate within the next 6 months.
• No plans for extended travel (>2 weeks) within the next 6 months.
• Live or work within 30 minutes of the AHWC (exceptions may be made at the discretion of the Study PI on a case-by-case basis for highly motivated subjects).
• Capable and willing to give informed consent, understand exclusion criteria, and adhere to study conditions.
• Have a primary care physician (or are willing to establish care with a primary care physician prior to study enrollment) to address medical issues which may arise during screening or study procedures.
• No contraindications to exercise or limitations on ability to be physically active.
• Ability to complete AM or PM exercise and complete 4 exercise sessions per week.
• Ability to adhere to eating and exercise protocols (meal timing, exercising in fasted and fed states)
• Willing and able to wear activity/sleep monitor for 7 consecutive days.
• Willing not to enroll in any other formal weight loss or physical activity programs over the next 6 months.
• For Females:
• Not currently pregnant or lactating.
• Not pregnant within the past 6 months.
• Not planning to become pregnant in the next 6 months

Exclusion Criteria:

• Diastolic blood pressure >100 mmHG, systolic blood pressure >160 mmHG, or resting heart rate >100 bpm as measured in duplicate on the screening visit after 5 minutes of rest in a seated position.
• Diabetes (fasting glucose >/=126 mg/dL or Hemoglobin A1C >/=6.5%) as measured during the screening visit.
• Undiagnosed hypo- or hyper-thyroidism (TSH outside of the normal range as measured during the screening visit) or history of uncontrolled thyroid disorder. History of thyroid disease or current thyroid disease treated with a stable medication regimen for at least 6 months is acceptable.
• Hematocrit, white blood cell count or platelets significantly outside the normal reference range as measured on the screening visit.
• Triglycerides >400 mg/dL as measured on the screening visit.
• LDL cholesterol >200 mg/dL as measured on the screening visit.
• Abnormal resting electrocardiogram (ECG) as measured on the screening visit: serious arrhythmias, including multifocal PVC's, frequent PVC's (defined as 10 or more per min), ventricular tachycardia (defined as runs of 3 or more successive PVC's), or sustained atrial tachyarrhythmia; 2nd or 3rd degree A-V block, QTc interval > 480 msec or other significant conduction defects.
• Presence or history of any metabolic or chronic health problems which would affect appetite, food intake, energy metabolism, or ability to participate in exercise: CVD, peripheral vascular disease, cerebrovascular disease, significant cardiac arrhythmias or cardiac valve disease, diabetes, uncontrolled hyper- or hypothyroidism, uncontrolled hypertension, cancer (within the last 5 years, except skin cancer or other cancers considered cured with excellent prognosis), HIV infection, significant renal, musculoskeletal, neurologic, hematologic, or psychiatric disease.
• Significant gastrointestinal disorders including: chronic malabsorptive conditions, peptic ulcer disease, Crohn's disease, Ulcerative Colitis, chronic diarrhea, or active gallbladder disease.
• Significant pulmonary disorders including: chronic obstructive pulmonary disease (COPD), interstitial lung disease, cystic fibrosis, or uncontrolled asthma.
• Symptoms suggestive of CVD: chest pain, shortness of breath at rest or with mild exertion, syncope.
• Regular use of prescription or over-the-counter medications known to significantly impact appetite, weight, or energy metabolism (e.g., appetite suppressants, lithium, stimulants, anti-psychotics, tricyclic antidepressants).
• Use of medications that would impact ability to achieve age-predicted maximum heart rate (e.g. beta blockers).
• Regular use of systemic steroids (other than Oral Contraceptive Pills).
• Regular use of obesity pharmacotherapeutic agents within the last 6 months.
• Current alcohol or substance abuse.
• Nicotine use (past 6 months).
• Current severe depression or history of severe depression within the previous year, based on DSM-IV-TR criteria for Major Depressive Episode. Score > 16 on the Center for Epidemiologic Studies Depression Scale (CES-D) will require further assessment by the Study MD to determine if it is appropriate for the subject to participate
• History of other significant psychiatric illness (e.g., psychosis, schizophrenia, mania, bipolar disorder) which in the opinion of the Study MD would interfere with ability to adhere to the exercise conditions.
• Weight loss >5% in past 3 months.
• Currently participating in or planning to participate in any formal weight loss or physical activity programs or clinical trials.
• Primary sleep disorders including but not limited to insomnia, obstructive sleep apnea, periodic limb movements in sleep, restless leg syndrome.
• Night-time shiftwork, rotating work, or other circadian disruptions and disorders (e.g., diagnosed delayed sleep-wake phase disorder).
• Self-reported long (>9.25h) or short sleep (<7h) that would make adhering the exercise conditions difficult.
• Regular use of prescription or over-the-counter medications known to affect sleep (e.g., benzodiazepines, doxepin, zolpidem, trazodone, diphenhydramine HCl, etc.).
• Regular use of melatonin (>/=2 days per week).
• High consumption of caffeine (>500 mg/d) or unwilling to abstain from caffeine consumption during run-in and in-patient protocol.
• Planned travel >1 time zone away 4 weeks prior to study start or at any time during study protocol.