Timing of Energy Availability on Menstrual Cycle Function

Inclusion Criteria: * participates in structured running or cycling exercise for at least 30 minutes on 3 or more days per week * accustomed to exercising for 90 minutes or more * has regular periods every 21-35 days * have not taken hormonal contraceptives for at least the past 6 months * is not currently or trying to become pregnant or breastfeeding, and has not been pregnant or breastfeeding in the past 12 months * does not currently have a diagnosis of a major menstrual cycle disorder (i.e., amenorrhea, polycystic ovary syndrome \[PCOS\], endometriosis, ovarian cancer, ovarian insufficiency, uterine or endometrial cancer) * does not currently have a metabolic disease (i.e., hypothyroidism, hyperthyroidism, Cushing disease, Addison's disease, diabetes, congenital adrenal hyperplasia) * does not currently have a major cardiovascular or respiratory disease Exclusion Criteria: * miss more than 7 consecutive days of aerobic activity (i.e., running, cycling, cross training) * demonstrate clinical low energy availability as defined as energy availability \<30 kcal/kg fat free mass * report menstrual cycle lengths \<21 or \>35 days in the first 2 months of at-home monitoring * do not demonstrate an anticipated rise in luteinizing hormone and progesterone in the first 2 months of at-home monitoring * begin taking a hormonal contraceptive * become pregnant * are diagnosed with a menstrual cycle disorder (i.e., amenorrhea, polycystic ovary syndrome \[PCOS\], endometriosis, ovarian cancer, ovarian insufficiency, uterine, or endometrial cancer) * are diagnosed with a metabolic disease (i.e., hypothyroidism, hyperthyroidism, Cushing disease, Addison's disease, diabetes) * are diagnosed with a major cardiovascular or respiratory disease * are unable to follow instructions for any of the procedures