Here is a study that got lots of press. Researchers studied over 54,000 Norwegians who were initially free of heart failure. The researchers asked the respondents how many times per week they have difficulty falling asleep, staying asleep, or woke unrefreshed. The results showed that in those that reported difficulty falling and staying asleep almost every night, and waking unrefreshed at least once a week, had a three fold increase in chance of developing heart failure. The researchers adjusted for factors that could affect the results, such as age, sex, marital status, education, shift work, blood pressure, cholesterol, diabetes, body mass index, physical activity, smoking, alcohol, any previous heart attack, depression and anxiety.
Now, before you panic, step back for a minute. These results support an association between insomnia and heart failure, but not that insomnia causes heart failure. Also, these patients were not examined before by sleep physicians and did not have sleep studies. Obstructive sleep apnea (OSA) could cause insomnia, and OSA is known to increase the risk of heart failure. So the insomnia reported by these study participants could be just a marker for undiagnosed OSA. This could explain the increased risk of heart failure, not the insomnia itself. And the study researchers did not adjust for OSA or other sleep related breathing disorders.
The authors theorize that if insomnia is linked to heart failure, then treating the insomnia may reduce risk of developing heart failure. I agree, as long as the cause of the insomnia is evaluated first.