Cardiometabolic health and the timing of habitual exercise in the All of Us Research Program

While the cardiometabolic benefits of exercise volume and intensity are well established, the clinical significance of exercise timing remains poorly understood, largely due to the limitations of short-term accelerometry. We leveraged minute-level heart rate data from 14,489 participants from the All of Us Research Program to define habitual exercise timing over a one-year period. Compared to daytime exercise, habitual morning exercise was associated with lower odds of coronary artery disease (OR 0.69; CI 0.55-0.87), hypertension (OR 0.82; CI 0.72-0.94), type 2 diabetes (OR 0.70; CI 0.58-0.85), hyperlipidemia (OR 0.79; CI 0.69-0.90), and obesity (OR 0.65; CI 0.55-0.77). These associations were independent of total physical activity volume and remained consistent across hour-of-day analyses, with the lowest risk nadir occurring between 07:00-08:00 for coronary artery disease. These findings suggest that exercise timing may represent a distinct, underappreciated dimension of exercise behavior linked to cardiometabolic health.