Inclusion Criteria:
* Female or Male
* Age 18-55 years
* Body Mass Index 25-40 kg/m2
* Physically inactive: defined as self-reporting \<150 minutes per week of physical activity at moderate intensity or greater on a regular basis over the past 3 months.
* No self-report of acute or chronic disease (cardiovascular disease (CVD), diabetes, gastrointestinal disorders and orthopedic problems in particular)
* No plans to relocate within the next 15 months.
* No plans for extended travel (\> 2 weeks) within the next 13 months
* Live or work within 30 minutes of the Anschutz Health \& Wellness Center (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, accept the randomized group assignment, and complete outcome measures.
* No contraindications to exercise or limitations on ability to be physically active.
* Willing to be randomized to either AM or PM exercise and complete 4 exercise sessions per week.
* Own a smart phone and willing to download and use text messaging for meal intake and other related assessments.
* Willing and able to wear activity/sleep monitor for 7-14 consecutive days.
* Willing not to enroll in any other formal weight loss or physical activity programs over the next 13 months.
* Fully vaccinated, or willing to be fully vaccinated, against COVID-19 prior to study enrollment (fully vaccinated is defined as at least 2 weeks post final vaccine dose).
* 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/interventions.
* For Females
* Not currently pregnant or lactating
* Not pregnant within the past 6 months
* Not planning to become pregnant in the next 15 months; sexually active women of childbearing potential may be enrolled if they have had a tubal ligation or use a reliable means of contraception
Exclusion Criteria:
* Diastolic blood pressure \> 100 mm HG or systolic blood pressure \> 160 mm HG.
* Resting heart rate \>100
* Diabetes (fasting glucose ≥126 mg/dL or Hemoglobin A1C ≥6.5%)
* Undiagnosed hypo- or hyper-thyroidism (Thyroid Stimulating Hormone (TSH) outside of the normal range) 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.
* Clinically significant abnormalities in hematocrit, white blood cell count or platelets.
* Triglycerides \> 400 mg/dL
* (Low Density Lipids) LDL cholesterol \>200 mg/dL
* Abnormal resting electrocardiogram (ECG): serious arrhythmias, including multifocal premature ventricular contractions (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 optimally participate in the exercise component including: 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 gastrointestinal disorders (described below), significant pulmonary disorders (described below), significant renal, musculoskeletal, neurologic, hematologic, or psychiatric disease.
* Significant gastrointestinal disorders including: 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, sleep, 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, cardio-selective calcium channel blockers).
* Regular use of systemic steroids (other than Oral Contraceptive Pills)
* Regular use of obesity pharmacotherapeutic agents within the last 6 months.
* Previous obesity treatment with surgery or weight loss device, except: (1) liposuction and/or abdominoplasty if performed \> 1 year before screening, (2) lap banding if the band has been removed \> 1 year before screening, (3) intragastric balloon if the balloon has been removed \> 1 year before screening (4) duodenal-jejunal bypass sleeve, if the sleeve has been removed \> 1 year before screening or 5) AspireAssist or other endoscopically placed weight loss device if the device has been removed \> 1 year before screening.
* Current alcohol or substance abuse
* Nicotine use (past 6 months)
* History of clinically diagnosed eating disorders including anorexia nervosa, bulimia, binge eating disorder. Score \>20 on the Eating Attitudes Test (EATS-26) or pattern of response on the Questionnaire on Eating and Weight Patterns (QEWP-5) suggestive of possible binge eating disorder or bulimia will require further assessment by the Study MD to determine if it is appropriate for the subject to participate in the study.
* Current severe depression or history of severe depression within the previous year, based on Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) criteria for Major Depressive Episode. Score \> 18 on Beck Depression Inventory (BDI) will require further assessment by the Study MD to determine if it is appropriate for the subject to participate in the study.
* 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 dietary or exercise interventions.
* Night-time shiftwork, rotating work, irregular sleep/wake patterns or other scheduling constraints which may hinder ability to consistently exercise at specific times of the day.
* Urinary incontinence or retention (as per PI discretion based on whether degree of incontinence/retention may impact doubly labeled water measures).
* Weight loss \>5% in past 3 months.
* Weight gain \>10% in past 3 months requires assessment by PI to determine reason for weight gain and if it is appropriate for the subject to participate in the study.
* Weight loss of \>50 lbs in past 3 years for any reason except post-partum weight loss requires assessment by PI to determine reason for weight gain and if it is appropriate for the subject to participate in the study.
* Currently participating in or planning to participate in any formal weight loss, dietary modification, or physical activity/exercise programs or clinical trials.
