the purpose of this experiment is to determine if acute melatonin supplementation alters central and peripheral cardiovascular responses to a cold pressor test at rest and during dynamic exercise.

After IRB approval, experiments will take place in the Human Circulation Research Laboratory in the Department of Health and Exercise on the University of Oklahoma Norman campus. All subjects will complete a total of 3 visits. The first is a screening visit to provide informed consent and to ensure the potential participant meets all inclusion criteria and no exclusion criteria. Once consented and enrolled, participants will be familiarized with experimental protocols. On separate days (visits 2 and 3), participants will complete placebo (PLA) and melatonin (MEL) in a randomly ordered, single-blind, counter-balanced design. PLA or MEL treatment will precede testing by 30min to maximize absorption into the circulation (Bartoli, 2013).

All experiments (PLA/MEL) will begin with the subject lying supine for 5 min of quiet rest where baseline measurements are taken, followed by activation of the lower body negative pressure (LBNP) device causing suction at -20 mmHg. Following LBNP participants will be given 10 minutes of rest to recover all variables to pre-LBNP values. After which participants will be given a hand grip dynamometer to complete 7 minutes of exercise at an intensity of 20% MVC at a 1s contraction to 2s relaxation cycle paced by a metronome. During the final 2 minutes of the 7 minutes of exercise the LBNP device will be activated again to -20mmHg such that the participant will be completing LBNP during rhythmic grip exercise. After the completion of exercise, the subject will remain supine for 10min. Then the a Total Labile Signal (TLS) procedure will be conducted. Where the experimental limb is occluded using a blood pressure cuff for 5min. The cuff is then rapidly deflated. The lowest oxyhemoglobin measured during the occlusion is recorded. After the TLS procedure, the subject will be de-instrumented and will leave the laboratory.

Inclusion Criteria:

• Healthy men and women
• Women must be premenopausal with a regular menstrual cycle (26-30 days)

Exclusion Criteria:

• History of autonomic dysfunction
• Cardiovascular diseases
• Diabetes
• Tobacco/nicotine use
• Systolic Pressure >= 130 mmHg
• Diastolic Pressure >= 85 mmHg
• Regular melatonin use >= 1 use/week
• Allergy to melatonin
• Cardiometabolic medication use (e.g. anti-hypertensives, insulin-sensitizing, statins)
• Sex hormone replacement medical use (e.g. testosterone, estrogen, progesterone)
• Pregnancy

The is no plan to share IPD

Participants will be blinded in two trials. Practitioners and operators will not be allowed to communicate with the participants concerning whether they are receiving placebo/melatonin. Outcome assessors and statisticians may not be blinded to experimental allocation. Our placebo should satisfy two conditions of an ideal placebo: a) it should be indistinguishable from the experimental supplement to blinded participants, and b) it should be physiologically inert. In this study, 1 microliter McCormicks Pure Mint Extract will be mixed with 29 ml of filtered water and given to participants as a sublingual spray. This mixture will be non-caloric and will mimic the taste and sensation of the commercially available melatonin spray that will be used in the study (Onnit Labs Inc., Mint Flavor).