Women who are often sleepy during the day tend to have underlying conditions that raise their risk of heart attack and stroke, according to a large new study.
Based on data for more than 84 000 US women, researchers linked daytime sleepiness to a more than doubled cardiovascular risk, but they say sleep disorders and other illnesses are really to blame, making the drowsiness a symptom, not a cause.
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"This is what we thought was going on," lead author James E. Gangwisch told Reuters Health in an email. "We thought that it was most likely that the daytime sleepiness was associated with insufficient sleep, shift work, snoring, and sleep adequacy," which are themselves associated with metabolic disorders like diabetes that are risk factors for stroke and heart attack, he said.
Gangwisch led the study at the College of Physicians and Surgeons at Columbia University in New York.
He and his coauthors analysed data from 84 003 women in the Nurses' Health Study II from 2001 to 2009. In the first year, the women answered a questionnaire that asked about sleep duration, disturbances, snoring and shift work.
One question asked how often a woman felt her daily activities were affected because she felt sleepy, and responses could range from "rarely" or "never" to "almost every day".
The researchers kept track of other factors like shift work, aspirin use, diabetes and high blood pressure every two years until 2009.
By that time, five hundred of the women had been diagnosed with heart disease or stroke.
Women who reported being sleepy during the day almost every day, which was five percent of the total group, were almost three times as likely to have been diagnosed with heart disease as those who were almost never sleepy during the day.
The women who were often drowsy were also more likely to have unusually short or long sleep durations, to have trouble getting adequate sleep, to snore, do shift work and to suffer from high blood pressure, diabetes, high cholesterol, obesity and depression.
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Once other sleep variables and diabetes, high blood pressure and high cholesterol were factored into the calculations, daytime sleepiness by itself no longer affected heart disease risk, according to the results published in the journal Sleep Medicine.
Gangwisch and his colleagues point out that they cannot determine whether there's a two-way relationship between poor sleep and other health conditions. Disturbed sleep, as happens with sleep apnoea, for instance, is thought to contribute to cardiovascular risk factors like high blood pressure.
On the other hand, they write, illnesses from depression to diabetes are thought to contribute to sleep disturbances as well as independently raising cardiovascular risk.
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