Inclusion Criteria:  18-35 old; men and women; equal numbers of women and men will be included. Body Mass Index (BMI) between 20.0 and 24.9 ("normal" weight). Physical activity history: inactive to habitual moderate physical activity level Sleep/wake history: habitual sleep duration between 7-9.25 hours. Altitude history: Potential subjects must have lived at Denver altitude or higher for at least 3 months prior to inpatient stay.  Exclusion Criteria:  Any clinically significant medical, psychiatric, or sleep disorder Use of prescribed medications/supplements/illicit drugs within one month prior to study History of shift work in year prior to study, or travel more than one time zone in three weeks prior to study. [NOTE: Subjects can be studied at a later date.] Women with history of prior gynecological pathology, <1 year post-partum, breast-feeding and/or pregnant. Non-English speakers will be excluded, as we do not have access to a translator who could rely accurate information to the participant for the consent or throughout the study.