People struggling with insomnia in the middle of their lives are at a greater risk of developing cognitive problems later in life, according to a new study involving 3,748 participants from Finland.

Those cognitive problems include issues with memory, concentration, and learning ability, researchers report – and the longer the insomnia lasts for, the worse these brain functions are likely to be as the years roll on, whereas if insomnia symptoms ease, cognitive function tends to stay healthier in later life.

We already know that our mental (and physical) health depends on a decent amount of sleep. However, few studies cover the length of time that this one does, with follow-up surveys carried out between 15-17 years after the original assessments of participants took place.

As a result, participants who were in mid-life and employed at the outset of the study had retired at the time of follow-up, having reached the age of statutory retirement, or for reasons of disability.

"Our results showed that insomnia symptoms already in working age can increase the risk of cognitive decline in retirement age," researchers from the University of Helsinki explain in their paper.

"The analysis showed that increased sleeping complaints were related to more severe problems in subjective cognitive function."

The study doesn't go into depth about the reasons for the connection. Previous studies have looked at the possibility that the waste-clearing system that operates in the brain during sleep, or the memory consolidation effects of REM sleep, could have an impact on cognitive function in the long term in people who sleep poorly.

The researchers adjusted for other health factors known to be linked to cognitive decline in old age. They include high blood pressure, high cholesterol, obesity, diabetes, depression, and a low level of physical activity.

Picking up on insomnia and treating it earlier could potentially stave off brain health issues and even diseases such as Alzheimer's later on in life, the study authors say – although the research isn't enough to conclusively show causation.

In other words, we don't know for sure that insomnia is what's causing the increased risk of cognitive decline, although the association seen certainly warrants future investigation.

"Early detection of insomnia symptoms already in mid-life could be a potential intervention point to improve sleep quality and prevent cognitive decline in later life," the researchers explain.

"These actions might save public funds and improve one's wellbeing, adding quality-of-life years in the context of aging."

The team points out that there are numerous ways to improve the quality of our sleep, including getting into a more regular sleep rhythm, making sure our sleep environment is well managed (in terms of temperature and lighting), and checking our eating and drinking habits (including coffee consumption, for example).

There are some limitations to be aware of with regards to the research. The study relied on self-reporting rather than objective tests, so the data is based on how aware participants were of their condition and how honest they were about it. Also, only the second, follow-up survey asked about cognitive issues.

However, there's enough evidence in the results here – collected as part of the Helsinki Health Study – to suggest an association that could be useful for future studies and for health assessments. It seems that insomnia has both long-term as well as short-term effects on the brain.

"In subsequent studies, it would be interesting to shed further light on, for example, whether the treatment of insomnia can also slow down the development of memory disorders," says University of Helsinki medical sociologist Tea Lallukka.

The research has been published in the Journal of Aging and Health.