The goal of this clinical trial is to determine how changes in exercise and psychosocial stress may influence the risk of menstrual cycle irregularities in female runners and cyclists. The main questions it aims to answer are:

• Does the implementation of exercise or psychosocial stress effect circulating reproductive hormones (i.e., estradiol, progesterone, luteinizing hormone) and menstrual cycle length?
• Is there an additive effect of combined exercise and psychosocial on circulating reproductive hormones (i.e., estradiol, progesterone, luteinizing hormone) and menstrual cycle length?

Participants will be asked to do the following over the ~3 month enrollment period:

• attend a laboratory visit at the beginning and end of the study to have their resting metabolic rate, aerobic fitness, and body composition tested
• monitor their menstrual cycle length, daily perceived stress levels, physical activity, and diet each month
• provide several urine and saliva samples each month
• either maintain their usual physical activity and lifestyle habits (control group), increase the duration of their weekly running or cycling mileage by 30% (exercise stress group), complete cognitive function tasks designed to be stressful (psychosocial stress group), or increase the duration of their weekly running or cycling mileage by 30% and complete cognitive function tasks designed to be stressful (exercise + psychosocial stress group) during the final month of enrollment.

Researchers will compare control, exercise stress, psychosocial stress, and exercise + psychosocial stress groups to see if there is an effect on circulating reproductive hormones (i.e., estradiol, progesterone, luteinizing hormone) and menstrual cycle length.

Inclusion Criteria:

• participates in structured running or cycling exercise for at least 30 minutes on 4 days per week or more
• have regular periods every 21 to 35 days
• have not used hormonal contraceptives for at least the past 6 months
• are not currently or trying to become pregnant or are breastfeeding, and have not been pregnant or breastfeeding for the past 12 months
• have never been diagnosed with a menstrual cycle disorder (e.g., menorrhagia, amenorrhea, dysmenorrhea, polycystic ovary syndrome [PCOS], endometriosis, premenstrual dysphoric disorder [PMDD], menstrual migraines, ovarian cancer, ovarian insufficiency, uterine or endometrial cancer)
• have never been diagnosed with a metabolic disease (e.g., hypothyroidism, hyperthyroidism, Cushing disease, Addison's disease, diabetes)

Exclusion Criteria:

• missing > 4 consecutive days of aerobic exercise (i.e., running, cycling, cross training) if assigned to the exercise intervention groups
• they indicate they "Could not participate at all due to a health problem" or have had to modify their exercise training, felt their injury, illness, or other health problem has affected their exercise performance, or experienced symptoms/ health complaints greater than "To a minor extent" on the Oslo Sports Trauma Research Center Questionnaire (exercise training intervention group)
• demonstrate clinical low energy availability as defined as energy availability <30 Kcal/kg fat free mass in the first two months of at-home monitoring
• report menstrual cycle lengths <21 days or >35 days in the first two months of at-home monitoring
• do not demonstrate an anticipated rise in progesterone levels during the latter half of their cycle and luteinizing hormone levels at mid-cycle compared to tests taken during the first few days after menses in the first two months of at-home monitoring
• begin taking a hormonal contraceptive
• become pregnant
• are diagnosed with a menstrual cycle disorder (e.g., menorrhagia, amenorrhea, dysmenorrhea, polycystic ovary syndrome [PCOS], endometriosis, premenstrual dysphoric disorder [PMDD], menstrual migraines, ovarian cancer, ovarian insufficiency, uterine or endometrial cancer)
• are diagnosed with a metabolic disease (e.g., hypothyroidism, hyperthyroidism, Cushing disease, Addison's disease, diabetes), 10) are diagnosed with a major cardiovascular disease, respiratory disease, or musculoskeletal injury
• are unable to follow instructions for any of the procedures