Top Things to Know: Time of Day as a Biological Variable in Cardiovascular & Kidney Disease Research
Updated: August 20, 2026
Prepared by David Pollock, PhD, FAHA, and Michelle Gumz, PhD, FAHA
- Time‑of‑day is a fundamental biological variable that needs to be considered in preclinical and clinical research as well as clinical practice.
- Humans and rodents operate on opposite circadian schedules, creating the potential for translational mismatches between active and inactive periods.
- The timing of lifestyle behaviors (sleep, food intake, activity, light) strongly shapes cardiometabolic health.
- Circadian disruption from shift work, jet lag, and inconsistent sleep-wake timing (i.e., social jet lag) is a significant risk factor for cardiometabolic and kidney disease.
- Molecular clocks exist in nearly every cell type and regulate a broad range of physiological functions.
- Proper experimental design requires deliberate timing of measurements and transparent reporting in order to strengthen rigor and reproducibility.
- Chronotherapy—timing treatment to biology—has real clinical potential.
- Current tools for assessing circadian phase in humans are limited, and better biomarkers are needed.
- Lack of awareness and lack of standardized circadian metrics are major barriers to progress in understanding fundamental mechanisms of physiology and pathophysiology.
- Future advances will rely on integrating circadian science with research technologies, clinical trials, electronic health records, and clinical care.
Citation
Pollock DM, Martino TA, Knutson KL, Panda S, Chaix A, Paul K, Shimbo D, Gumz ML; on behalf of the American Heart Association Kidney in Heart Disease Science Committee of the Council on the Kidney in Cardiovascular Disease; Council on Cardiovascular and Stroke Nursing; and Council on Lifestyle and Cardiometabolic Health. Time of day as a biological variable in cardiovascular and kidney disease research: a scientific statement from the American Heart Association. Arterioscler Thromb Vasc Biol. Published online August 20, 2026. doi: 10.1161/ATV.0000000000000198