Influence of Melatonin on Cardiovascular and Thermoregulatory Responses to Stress

Inclusion Criteria: * Must be between the ages of 18-70 years old. * All subjects will be required to abstain from exercise and caffeine for 12 h, and alcohol for 24 h prior to the experiment. * BMI must be \<30 kg/m2. * Menstruating women will initially be tested during their early follicular phase (2-5 days after initiating menstruation) or during low hormone phase (2-5 days after initiating menstruation) if on oral contraceptives to control for potential impact of sex steroids. Post-menopausal females (\>5 years) will also be included. Females must have an intact uterus and at least one ovary. Use of hormonal replacement therapy will be allowed. Exclusion Criteria: * Circadian rhythm sleep disorders * High obstructive sleep apnea diagnosis determined by STOP-BANG. * History of meeting Diagnostic and Statistical Manual of Mental health (DSM-V) criteria of major psychiatric disorder * Unstable or serious medical conditions (heart failure, diabetes, cardiovascular disease, etc.) * Current, or use within past month, of psychoactive (other than stable treatment with antidepressant), hypnotic, stimulant or analgesic medication (except occasional non-narcotic analgesics), beta blockers, or alpha blockers * Shift work or other types of self-imposed irregular sleep schedules * Habitual smoking (6 or more cigarettes per week) * Habitual alcohol consumption (more than 2 alcoholic drinks per day) * Pregnancy or breast feeding