William C. Shiel Jr., MD, FACP, FACR
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
Melissa Conrad Stöppler, MD
Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
In this Article
- What are biologic rhythms?
- How does the "body clock" affects symptoms of illness?
- What are examples of specific diseases affected by biorhythms?
- Heart attack
- High blood pressure (hypertension)
- Hay fever (allergic rhinitis)
- Can the "body clock" affect diagnostic testing?
- Can drug therapy be matched to the "body clock?"
High blood pressure (hypertension)
Hypertension or high blood pressure is a silent killer. Early high blood pressure produces no symptoms or discomfort. However, untreated long-term high blood pressure can lead to strokes, heart diseases, kidney disease, and eye damage. Medical scientists now know that a person's blood pressure varies according to the time of day (circadian rhythm). Such variations may have important implications in the diagnosis, treatment, and monitoring of patients with high blood pressure.
It is important to understand a critical distinction: Circadian rhythms do not cause clinical hypertension in normotensives (people with normal blood pressure). Circadian rhythms do, however, result in a cyclic worsening of hypertension in hypertensive patients.
In the majority of individuals - normotensive and hypertensive - blood pressure rises rapidly in the early morning hours, the time when most individuals wake up and begin their day. This morning rise in blood pressure is not caused by the act of getting out of bed and beginning activity. It is a genetically preset system that automatically increases a person's blood pressure at that time.
For example, if a person goes to sleep at around 10 p.m., beginning in the very early morning - sometime between 3 a.m. and 5 a.m. – the blood pressure begins to rise and continues to rise through the morning.
This increase, which is accompanied by an increase in heart rate, corresponds to the following biologic changes, which are also governed by circadian rhythms:
- Increased secretion of catecholamines, particularly norepinephrine, into the bloodstream.
- Increased plasma renin activity.
Catecholamines, norepinephrine, and renin are natural hormones produced by the kidneys and adrenal glands (small glands located near the top of both kidneys). These hormones cause tightening of blood vessels in the body (vasoconstriction). Vasoconstriction causes resistance to blood flow and elevates blood pressure.
The vasoconstricting effects of catecholamines may also be enhanced in the morning by the high levels of certain hormones which interact with catecholamines and increase their effects. Many high blood pressure medications are designed to counteract the vasoconstrictive effects of these hormones.
In the late morning or early afternoon, the natural rise in blood pressure reaches its peak. After that, blood pressure declines, falling 15 to 20 mmHg between about 8 p.m. and 2 a.m., the time at which blood pressure is normally at its lowest point.
The blood pressure pattern just described is typical of some individuals, whether the individual is hypertensive or normotensive. These people are referred to as "dippers." In other individuals (called "nondippers"), there is no nightly decline in blood pressure. The blood pressure in these people remains high, even during sleep.
Disease of the heart and blood vessels (cardiovascular disease), including hypertension and the syndromes of ischemic heart disease (angina, heart attack, sudden death), also follow circadian rhythms. This pattern makes sense when you consider that heart rate and blood levels of enzymes and catecholamines that strongly influence blood pressure and heart function follow circadian rhythms.
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