People who find it hard to sleep may have an increased chance of developing heart failure, a study has shown.
Three symptoms of insomnia more than tripled the risk in a group of 54,279 people aged 20 to 89.
Scientists stress that they have only found an association and cannot be sure there is a causal link, but they suspect insomnia may trigger stress responses that affect the heart.
Heart failure occurs when the heart is too weak to pump blood around the body efficiently, leading to symptoms of exhaustion and breathlessness that can interfere with daily life.
Study leader Dr Lars Laugsand, from the Norwegian University of Science and Technology in Trondheim, said: "We related heart failure risk to three major insomnia symptoms, including trouble falling asleep, and not waking up feeling refreshed in the morning.
"In our study, we found that persons suffering from insomnia have an increased risk of having heart failure. Those reporting suffering from all three insomnia symptoms simultaneously were at considerably higher risk than those who had no symptoms or only one or two symptoms."
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Dr Laugsand's team collected data from men and women enrolled into the Nord-Trondelag Health Study whose health was monitored for an average of around 11 years. During this period, a total of 1,412 cases of heart failure were recorded.
Participants were questioned about their quality of sleep. Those who had difficulty getting to sleep and staying asleep, and who failed to wake feeling refreshed in the morning more than once a week, were more at risk of heart failure than people experiencing these symptoms only rarely or not at all.
A statistically significant 353% increase in risk was associated with having all three insomnia symptoms.
When the results were adjusted to take account of depression and anxiety, the risk was raised more than four-fold.
Writing in the European Heart Journal, the authors concluded: "We found a moderate risk increase related to the individual insomnia symptoms. However, the risk among those with all the three insomnia symptoms simultaneously was particularly high even after adjustment for established cardiovascular risk factors and psychological distress."
Dr Laugsand said further research was needed to establish whether insomnia was causally linked to heart failure.
"It is still unclear why insomnia is linked to higher heart failure risk," he said.
"We have some indications that there might be a biological cause, and one possible explanation could be that insomnia activates stress responses in the body that might negatively affect heart function. However, further research is also needed to find the possible mechanisms for this association."
He stressed that insomnia could be treated using simple strategies such as educating people about sleeping habits, or providing psychological and medical therapies.
"Evaluation of sleep problems might provide additional information that could be used in prevention of heart failure," he added.
British expert Dr Tim Chico, consultant cardiologist at the University of Sheffield, said: " This is an 'association study'; it links insomnia to heart failure, but does not prove that insomnia causes heart failure or vice versa.
"Studies like this raise interesting suggestions that need further work to examine. Insomnia is a very unpleasant condition, but there are effective lifestyle changes that can reduce it, such as weight loss and exercise.
"Luckily, many of the things that reduce the chance of heart failure also reduce insomnia: good diet, exercise, weight loss and not smoking."