The Power of Circadian Rhythms

May 8, 2000 -- Circadian rhythms -- our 24-hour biological cycles -- govern more than our waking and sleeping. They affect when we are born, when we die, and how we pass the days in between.

Our body's rhythms are governed by a "master clock" located in a tiny region of the brain called the suprachiasmatic nucleus. It works much like a conductor, striking up one section of the body's orchestra as another quiets down, taking its main cue from light signals in order to stay in sync with the 24-hour day.

Our body's hormones surge and ebb to this maestro's invisible wand. Even our cells grow faster during certain hours of the day.

Here are just a few of the ways our circadian rhythms orchestrate our lives:

  • Night often brings life. Evolution has timed hormones to trigger labor and birth at night, when the mother and her newborn would be less vulnerable to predators, according to Darwinian (evolutional) theory. Indeed, studies have shown that natural deliveries occur more frequently in the hours after midnight than during the afternoon.
  • Morning often brings death. Our blood pressure is lowest around 3 a.m. When dawn breaks and we rouse ourselves from bed, our blood pressure rises sharply, increasing the risks of heart attack and stroke between 8 a.m. and noon.
  • Asthma often strikes hardest at dawn. One possible reason is that our bodies produce less cortisol, an anti-inflammatory steroid, during the night.
  • Allergies often flare when we wake. Sneezing and runny noses tend to be worse in the morning for 70% of allergy sufferers.

Findings such as these have led to the budding field of chronotherapy, in which doctors time medicines to the body's natural rhythms. For instance, allergy sufferers can ease their morning symptoms by taking a long-acting antihistamine late at night. Asthmatics can reduce their risk of a morning attack by taking the drug theophylline at night.

There's even reason to believe that the circadian clock affects our cells' tolerance for toxins. By manipulating its timing and dosage, doctors hope to lessen the side effects of cancer chemotherapy.

"We have found over 35 medical conditions that are affected by the body's internal clock," says Michael Smolensky, PhD, professor of environmental physiology at the University of Texas-Houston School of Public Health and co-author of The Body Clock Guide to Better Health. "That concept is revolutionary, and there's more coming."

Sarah Yang is a San Francisco reporter for WebMD.