The goals of this study are to uncover the influence of diet on the human circadian timing system. The protocol is a 46-day (28 outpatient days, 18 inpatient days over two 9 day visits) randomized cross-over study designed to elucidate the speed of entrainment in response to a high-fat diet.

Synchronization of the timing of biological processes and behaviors with the 24-hour light-dark cycle is fundamental to almost every physiological process, cognitive function, and overall health. As the average circadian period in humans is ~9 min longer than the 24-h day, and artificial lighting allows a further delay in the clock, circadian entrainment must occur every day. This makes the process of circadian entrainment of the internal clock with the external environment a commonly overlooked process, but one that has serious ramifications if impaired.

In rodent models, high-fat diets have been shown to disorganize the circadian timing system. There has been limited examination, however, of how composition of diet acts on the circadian timing system in humans. The ability to entrain to a new light schedule has been briefly examined after a high-fat diet in rodent models.When mice are exposed to a 6-h shift in the light-dark cycle and fed a diet composed of ~50% fat for 3 weeks, they experience a 20% slower rate of re-entrainment to the new light cycle as compared to mice fed a low-fat diet. Further, these mice had an attenuated response to a phase-advancing light pulse. Moreover, simply providing a high-fat diet to mice results in a lengthened circadian period and a shift in ad libitum eating patterns into the typical rest phase.

The goals of this study are to uncover the influence of diet on the human circadian timing system. The protocol is a 46-day (28 outpatient days, 18 inpatient days over two 9 day visits) randomized cross-over study designed to elucidate the speed of entrainment in response to a high-fat diet. Participants will be provided 3 weeks of a high-fat diet (2-weeks outpatient, 1-week inpatient) of 50% calories from fat. The protocol includes two types of data collection; ambulatory monitoring on strict sleep-wake schedules with either high-fat or low-fat meals provided by the study team and precise in-laboratory measurements of circadian timing, entrainment, and other physiological markers of sleep and cardiometabolic health.

1. Ambulatory Monitoring: Participants will maintain a consistent 14-day at home 8h sleep schedule at habitual times before both laboratory visits to ensure 1) subjects are not sleep restricted and 2) that they are obtaining the same light-dark schedule prior to each laboratory visit for stable entrainment; verified by actigraphy, sleep logs, and call-ins to a time stamped recorder. Drugs, medications, caffeine, alcohol, and nicotine will be proscribed during this time and toxicology analysis will be performed upon in-laboratory admission.
2. Outpatient diet: For 2-weeks prior to each protocol, participants will consume an outpatient isocaloric diet designed to meet individual daily energy requirements; diet designed and prepared by investigators. Diets will either be designed to be high in calories from fat (50% fat, 35% carbohydrate and 15% protein; 33% of each mono, poly and saturated fat) or low-fat (30% fat, 55% carbohydrate and 15% protein) (randomized-crossover design). The diet will consist of a breakfast, lunch, dinner, and snack and participants will either come to the laboratory ~3 days or staff will deliver meals. Participants will be instructed to only consume the food provided during these 14-days. Adherence to the diet will be captured via tracking meals using a mobile food tracking application (MealLogger AppTM (Wellness Foundry, New York)).
3. Inpatient Protocols: Participants will be admitted to an individual room free of external time cues (e.g., clocks, radios, computers, visitors and sunlight). Room temperature is maintained at 21 - 22.2 °C and light intensity set at ≤5 lux during scheduled constant posture procedures, room light (400 lux) during entrainment wakefulness and 0 lux (darkness) during scheduled sleep opportunities. Participants will be instrumented for full polysomnography (PSG), electrocardiogram, and a rectal thermistor for continuous core body temperature measurement. An 18-gauge IV catheter will also be inserted into the forearm and connected to a triple-stopcock manifold via an IV loop with a 12- foot small-lumen extension cable through which blood sampling can continue in the next room without disturbing sleep.

After instrumentation, participants will be given a sleep opportunity and will awakened to dim-lighting and maintaining a constant posture protocol for accurate assessment of circadian phase, resting metabolic rate, and other physiological outcomes. After the constant posture protocol, participants' light-dark cycle will be changed for 6 identical days, followed by a second constant posture protocol for re-assessment of circadian phase. This protocol will be repeated while participants are provided the other cross-over meal. Investigators/nurses will be present in the lab or in a central control room 24 h per day to monitor subject health, data acquisition, provide meals, collect biologic specimens, perform tests, and record sleep. A physician is always on call when a participant is in the laboratory. An extensive series of protocols and checklists and team practices are used to ensure uniformity in execution of standard procedures.

Energy content of diets will be designed to meet individual daily energy requirements. Dietitians will prepare isocaloric meals containing macronutrient contents of high or low-fat diet and no caffeine. Caloric intake will be the same caloric and macronutrient composition for each day of laboratory study and will be provided as miniature snacks frequently during each constant posture protocol (e.g., ¼ turkey & cheese sandwich, juice and water).

Inclusion Criteria:

• Healthy, lean men and women

Exclusion Criteria:

• Major dietary restrictions (such as entirely dairy-free and/or meat-free diets)

• A body mass index (BMI) of 18.5< [BMI] < 24.9 kg/m^2 and a waist circumference <94/80cm

• Currently psychiatrically/psychologically unsuitable for participation

• Drug/alcohol use, including smoking

• Medication/drug use, including prescribed and over-the-counter medications

• History of working irregular day and night hours, regular night work, or rotating shift work for the 1 year prior to the study.

• Traveled across more than 1 time zone during the 3 months prior to the study

• Currently consuming a habitual high-fat diet

• Chronobiologic and sleep disorders

• Diseases of the Cardiovascular System

• Metabolic Syndrome; Two or more of these factors will be excluded from the study:

  1. HDL cholesterol of less than 40 mg/dL in men or less than 50 mg/dL in women;
  2. systolic blood pressure>135 mmHg or diastolic blood pressure>85 mmHg;
  3. Fasting blood glucose ≥ 100 mg/dL;
  4. Triglycerides ≥ 150 mg/dL.

• Pre-Diabetes/Diabetes

• Hypertension

• Disorders of the Respiratory System

• Disorders of the Kidney and Urinary Tract

• Infectious Diseases

• Disorders of the Gastrointestinal System

• Disorders of the Immune System

• Disorders of the Hematopoietic System

• Neoplastic Diseases

• Endocrine and Metabolic Diseases

• Neurologic Disorders

• Subjects must not be currently participating in another research study that would influence their safe participation in our study

As it will be easy to discern meal fat content, participant blinding is not possible. Participants will be blinded to clock time and light-dark cues and statisticians will be blinded of condition.