Heart attacks often occur in the morning. Epileptic seizures peak in the late afternoon. Asthma attacks get worse and more deadly between 11 p.m. and 3 a.m.

Researchers are finding that circadian rhythms, which cycle every 24 hours or so, drive virtually every system in the human body, from circulation and cognition to metabolism, memory and mood. And they play a big role in determining when we are most vulnerable to disease.

Chronobiology—the study of these internal clock mechanisms—has exploded in recent years thanks in part to the discovery of specific genes, to which scientists have given names like Clock, Period and Cryptochrome. Those genes help keep our biological systems in sync with light and darkness, which makes for a rush hour of chemical changes at dawn and dusk.

Understanding biorhythms is helping doctors direct treatments, including the best times to take various medications. It is also suggesting new treatment strategies, such as adjusting the light in nursing homes to help people sleep better.

Of course, people often disrupt their circadian cycles with jet travel, erratic work schedules, watching TV and Web surfing long into the night. That can raise the risk for a variety of health problems, including heart disease and diabetes.

The body’s master time keeper is a group of neurons in the hypothalamus, located behind the eyes, called the suprachiasmatic nucleus, or SCN. In darkness, the SCN prompts the pineal gland to release melatonin, the hormone that facilitates sleep. Other chemical changes reduce body temperature, blood pressure and heart rate, all of which are at their lowest overnight.

Other systems are highly active at night. Stomach-acid production peaks between 10 p.m. and 2 a.m., digesting the evening meal and possibly exacerbating heartburn. The liver dumps glucose into the bloodstream just before dawn. Leptin, a hormone that damps appetite, is high, so people tend to be least hungry when they wake up. And the immune system may be overactive, inflaming airways in asthma sufferers and swelling arthritic joints.

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