This study will test will how eating on a particular daily schedule may effect energy, weight gain or loss, and body temperature.

Although there have been a large number of studies in humans on the effects of food intake at night, very few studies directly address the hypothesis that inappropriately phased eating or snacking (i.e., at night) in humans disrupts metabolism and respiratory quotient (RQ) patterns and there are no studies that we are aware of that directly measure circadian clock phase relative to timing of food intake while evaluating metabolism. This may be critical, because it is well established that the timing of food intake can also modulate circadian clock phase.

Hypothesis: Food consumption in the subjective night (e.g., 20:00 - 02:00) will result in a different circadian metabolic profile measured by RQ) than food consumed in the subjective day.

Aim 1: To determine if there are daily rhythms of switching between lipid and carbohydrate metabolism in humans that are altered by the timing of food intake. Aim 2: To determine if timing of food intake results in "internal desynchronization" between the metabolic rhythms (RQ) and the rhythm in core body temperature (a marker of central circadian phase).

Aim 3: To determine if body composition has measurable impact on the circadian regulation of metabolism.

In this study we will test human subjects in a specially designed whole-room indirect calorimeter where energy expenditure and RQ will be monitored by indirect calorimetry continuously over 56 hours. Circadian phase and amplitude will be assessed by continuously recording the core body temperature rhythm using the Vital Sense Integrated Physiological Monitoring System in which subjects swallow a telemetry capsule that transmits core body temperature to a data acquisition module. In consultation with a nutritionist at Vanderbilt, we will use a cross-over design in which daily diets have the same caloric and nutritional value but in which the subjects consume the calories that would normally be breakfast as snacks consumed in the late-evening.

Inclusion Criteria:

• Be able to understand the study, provide written informed consent (in English), and be able to fill out the questionnaire
• Be male or female older than 18 years of age;
• Have a normal BMI (20-25) or be obese (BMI more than 30);
• Have a normal basal glucose level (70-100 mg/dL)
• If female of childbearing potential, have a negative pregnancy test on study day;

Exclusion Criteria:

• Be pregnant or lactating;
• Have known sleep, metabolic (e.g., diabetes), or gastro-intestinal disorders except obesity;
• Had alcohol less than 24 hours before admission;
• Require assistance with activities of daily living;
• Have difficulty swallowing
• Be unable to complete a food and sleep diary
• Be smokers