Inclusion Criteria:

• All Participants (including returning participants)
• * Adult participants aged greater than or equal to 18 years at the time of enrollment.
• * Able to provide written informed consent.
• * Willing and able to complete study procedures.
• Fatigued Participants
• -In addition to the above, must experience fatigue for more than a month.

Exclusion Criteria:

• Exclusion from study entry:
• * Not fluent in English.
• * Have medical health issues that create additional and substantial adverse risks related to study procedures. Medical examples are the acute complications of medical disease, such as unstable hypertension, diabetic ketoacidosis, symptomatic hyperthyroidism, and unstable angina.
• * Have a psychiatric condition that prevents participant from being able to reliably report the fatigue experience (e.g., a psychotic disorder with severely disorganized thinking);
• * Current substance use disorder within the last five years or positive urine toxicology results at enrollment and before Stage 2. Prior marijuana use within the past five years will not be an exclusion.
• * Categorized as a high-risk drinker (greater than or equal to 5 drinks/day and greater than or equal to 15 drinks/week for men, greater than or equal to 4 drinks per day and greater than or equal to 8 drinks per week for women). ( Dietary Guidelines for Americans 2015-2020", U.S. Department of Health and Human Services and U.S. Department of Agriculture).
• * NIH Employees.
• * Pregnant or lactating women.
• * Ongoing medical condition that is deemed by the Principal Investigator to interfere with the conduct or assessments of the study or safety of the participant.
• Exclusion from specific tests:
• * fMRI Technical Development Sub-Study
• * Implanted cardiac pacemakers, metal aneurysm clips;
• * Broken bones repaired with metal pins, screws, rods, plates;
• * Prosthetic eye implants;
• * Transdermal medications or infusion pumps;
• * Bullet fragments or other metal pieces in body from old wounds;
• * Significant work exposure to metal particles;
• * Clinically relevant claustrophobia;
• * Unable to lie comfortably on back for up to 4 hours;
• * Left-handed. (Participants that are ambidextrous, will complete the Edinburgh Handedness Inventory).
• * EEfRT and Stroop Test
• * Severe essential hand tremor or severe hand pain (e.g., severe arthritis or severe carpel tunnel) that makes it difficult to perform rapid keyboard presses will be an exclusion from the EEfRT and Stroop Test.
• * Color-blindness, verified by scoring greater than or equal to 14 on the Ishihara card test, will exclude a participant from taking the Stroop test. The Ishihara card test will be administered only if the participant states he or she is color-blind.
• * Tilt Table Test
• --Beta blocker medications.
• * CPET
• * Any neuromuscular conditions (e.g., multiple sclerosis, Parkinson s disease) with significant mobility impairment that prohibits treadmill or bike performance, or any medical condition (e.g., congestive heart failure, coronary artery disease, chronic obstructive pulmonary disease \[COPD\], severe osteoarthritis and poorly controlled asthma) that would make it risky for the participant to exercise (e.g., exercise-induced angina and asthma), will exclude a participant from the CPET.