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Circadian Rhythms: Turns for the Worse

Understanding circadian rhythms may help doctors time treatments for optimum effect.

By Anita Slomski // Spring 2012
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Symptoms of certain diseases worsen at night, others during the day. Research is attempting to tease out the role of the circadian system in these patterns. Answers may lead to better therapy that is optimally timed to when body clocks are most receptive to therapeutic intervention.


Heart attacks occur most frequently between 6 a.m. and noon. Hormones called catecholamines are released in the morning to prepare the body for the physical stress of getting out of bed. The hormones increase the formation and stickiness of platelets, which clot around an atherosclerotic plaque that has ruptured, blocking blood flow.


The chronic inflammation and airway obstruction that characterize asthma are at their worst at 4 a.m. Airways are less open and greater numbers of white blood cells circulate to remove damaged cells and fight toxins. Excessive numbers of white blood cells signal chronic inflammation, which, in people with asthma, leads to narrowing of the bronchial tubes.


Temporal lobe epileptic seizures occur most often between 3 and 7 p.m., peaking around 5 p.m., whereas seizures that originate in other parts of the brain have different incidence patterns. Scientists don’t yet know why.


People with Alzheimer’s disease often experience “sundowning”—greater disorientation during late afternoon and early evening hours, possibly as a result of decreased melatonin production. Melatonin is a powerful antioxidant that protects neurons from the toxicity of the abnormal accumulation of beta-amyloid proteins in the brain, the hallmark of Alzheimer’s.


Heartburn arising at night tends to last longer, possibly because of circadian-related decreases in swallowing and saliva, which neutralize stomach acid.


Cellular circadian clocks regulate many aspects of cell function, including when cells divide and repair their DNA—both processes that go awry in cancer. Chemotherapy may be tolerated at higher doses and will be more effective when given at specific times of the day or night, depending on the type of cancer, researchers believe. To shrink a tumor more quickly, for example, it might be best to administer chemotherapy when an organ’s cells—including the abnormal ones—are actively dividing.


Circadian Rhythms: The Timekeepers Within

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Clock genes keep circadian rhythms in sync, coordinating cells’ essential work and possibly enhancing well-timed therapies.

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