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A population-based study of more than 52,000 Norwegian men and women suggests that sleepless nights may increase the risk of a heart attack.

People who struggled to fall or stay asleep almost every night were 30% to 45% more likely to have an acute MI, Lars E. Laugsand, MD, of the Norwegian University of Science and Technology in Trondheim, and colleagues found.

The more insomnia symptoms reported, the higher the risk (P=0.003 for trend), they wrote in Circulation: Journal of the American Heart Association.

Although the observation suggests just moderate relative risks, the authors estimated that insomnia may affect as many as one third of adults and symptoms are easily recognizable.

"Therefore, evaluation of insomnia might provide additional information in clinical risk assessment that could be useful in cardiovascular prevention," they suggested in the paper.

Management can be as simple as following sleep hygiene recommendations, although targeted pharmacological and nonpharmacological therapies may be needed to help with chronic insomnia, the researchers pointed out.

They analyzed questionnaire responses from 52,610 men and women living in a single county in Norway and followed for acute MI through national hospital and vital status registries in the Nord-Trøndelag Health Study.

During 11.4 years of follow-up, 2,368 incident acute MIs were recorded.

Difficulties initiating sleep almost every night were reported at baseline by 3.3% of the participants; trouble maintaining sleep almost every night by 2.5%; and nonrestorative sleep more than once a week by 8.0%.

People who struggled to fall asleep almost every night were 45% more likely to have an acute MI (adjusted hazard ratio 1.45, 95% confidence interval 1.18 to 1.80) compared with those who never experienced this sleep problem.

Trouble staying asleep nearly every night was associated with 30% elevated risk (adjusted HR 1.30, 95% CI 1.03 to 1.57).

Nonrestorative sleep two or more times a week predicted 27% elevated risk of acute MI (adjusted HR 1.27, 95% CI 1.03 to 1.57).

These results had been adjusted for age, sex, education, shift work, marital status, depression, anxiety, and heart disease risk factors, such as smoking and physical activity.

Sensitivity analyses further excluding the first five years of follow-up, acute MI not hospital verified, and those with chronic somatic disorders didn't have much impact.

Interestingly, excluding participants on sleep medications or sedatives strengthened the association between heart attack risk and trouble falling asleep.

"Possibly, this could indicate that sleep medication may reduce acute MI risk by reducing difficulties initiating sleep," the group suggested, but they cautioned that "this important question cannot be investigated properly in the present study."

Potential mechanisms include common risk factors between sleep disorders and heart disease, such as increased sympathetic activation and high blood pressure, they noted.

Sleep apnea, which is well-established as a cardiovascular disorder, wasn't assessed.

Other limitations of the study included lack of objectively assessed sleep quality through polysomnography; lack of data on duration of sleep; and inability to exclude the possibility of uncontrolled confounding.

Moreover, the results may not readily and directly generalize to populations living at lower latitudes, with different underlying heart attack risk or with different sleeping and circadian habits, the researchers cautioned.

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