Abstract Body: Objective: To determine the role of the circadian system on blood pressure (BP) reduction promoted by exercise. Methods: Nine (4 male/5 female) healthy adults (aged 23±2 y) completed a 30-hour circadian protocol in a dim light laboratory (<8 lux), where all behaviors and measurements were evenly distributed across the 24-hour cycle. Participants underwent five recurring 6-hour day-cycles of 2-hour sleep opportunities and 4-hour standardized wake episodes. Systolic, diastolic, and mean BPs, cardiac output (CO), and stroke volume were derived from beat-to-beat BP waveform (plethysmography). Heart rate (HR) was determined from an electrocardiogram (ECG), and systemic vascular resistance (SVR) was calculated (SVR = Mean BP/CO). Data were recorded for 10 minutes in supine at rest, before and after 30 minutes of moderate aerobic exercise (cycle ergometer, 30 min, 40% HR reserve). Circadian phases were calculated based on individual dim-light melatonin onsets. Mixed-model cosinor analyses tested if changes between before and postexercise had systematic rhythmicity with an underlying sinusoidal profile. Results: Systolic and mean BP change had a circadian rhythm (p<0.05) with the greatest decrease (-12 and -9 mmHg) at ~1pm and an amplitude of 11 and 8 mmHg, respectively, from its blunted response at ~1am. SVR change had a circadian rhythm with a greater decrease ~10am (see figure). No other variables displayed a circadian variation. Conclusion: In young healthy adults, the circadian system potentiates the hypotensive effect of exercise in the afternoon. However, the absence of this ability during the night may represent cardiovascular vulnerability in those who stay awake at that time (e.g., shift workers).