Influence of Caffeine on Psychomotor Vigilance and Carbon Dioxide Tolerance During Graded Hypercapnia

Inclusion Criteria: * Male or female, age 18-45 * Body mass index 18.5-30.0 kg m-2 * Body mass 50-100 kg * In good health as determined by the Office of Medical Support \& Oversight (OMSO) General Medical Clearance * Perform exercise at least 2 times per week * Willing to abstain from exercise and alcoholic beverages for 24 hours before each study visit. * Willing to abstain from caffeine for 12 hours prior to all study visits * Willing to abstain from caffeine for 12 hours after visits 3 and 4 * Willing to remain fasted (no food or fluid other than water) from lights out until after arrival at the laboratory the following morning on each study day (\~10 hours) * Have supervisor approval if permanent party military or a federal employee at U.S. Army Natick Soldier System Center Exclusion Criteria: * Females who are pregnant or planning to become pregnant during the study * Females who are surgically sterile * History of severe adverse reaction to caffeine (e.g., headache, dizziness, diarrhea, insomnia) * Taking dietary supplements, prescriptions, or over the counter medication, other than a contraceptive (unless approved by OMSO \& PI) * Abnormal blood count (For example: hemoglobin (Hb) outside of the typical normal values reported by LabCorp in accordance with OMSO (Normal \[Hb\] Males = 12.6-17.7 g/dL; Females = 11.1-15.9 g/dL) or hematocrit (Hct) outside of the normal ranges (Normal Hct Males = 37.5-51.0%; Females = 34.0-46.6%) levels, presence of abnormal blood chemicals (hemoglobin S or sickle cell traits) * Any history of pulmonary or cardiovascular disease * Current diagnosis of asthma (childhood asthma with no recurrence in the last 5 years ok) * Smokers or nicotine users (unless have quit \>1 month prior) * Current or recent respiratory tract or sinus infections (\< 1 month prior) * Current diagnosis of migraine or recurrent headaches (previous diagnosis with no recurrence in the last 5 years ok) * Any history of seizures * Any history of panic disorder * Blood donation in the previous 8 weeks * Positive Covid-19 test within the last month (based on self-report)