Connection between light levels and mental health -- climate change could also have an impact in the future

The results can be found in a study funded by the Research Council of Finland. The study was conducted as a part of the Climate Change and Health research programme. The aim of the study was to investigate the connection between changing light levels and mental health. It is expected that due to climate change, winters in Finland will become darker while summers will become brighter.

During the study, Kela's sick leave register was used to analyse the seasonal timing of a total of 636,543 sick leaves that were due to mental health reasons over a period of 12 years. The analyses examined whether the expected number of absences was above or below the expected number of sick leaves.

"Previous studies have found that some people experience so-called winter depression (seasonal affective disorder) during the dark season. In addition to the typical symptoms of depression, kaamos depression involves an increased appetite and weight gain along with excess sleepiness, which means sleeping for longer and feeling tired during the day. The symptoms of winter depression can often be alleviated through bright light therapy," says Timo Partonen, a Research Professor at the Finnish Institute for Health and Welfare.

Seasonal variation can increase workloads in the workplace and in health services particularly in the autumn, when the most common types of sick leaves -- absences due to depression, anxiety and sleep disorders -- are starting to occur often.

"It's also worth considering if there are other explanations for the phenomenon apart from a dark season. For example, is there an exceptionally high amount of psychosocial stress in the workplace during autumn, which then leads to an increasing number of sick leaves," says Professor of Psychology Marianna Virtanen from the University of Eastern Finland.

If climate change causes summers in Finland to become brighter and winters to become darker, the study suggests that depression, anxiety and sleep disorders could increase during the winter because of those changes. However, with the exception of sleep disorders, they could also become less prevalent during the summer. In the case of bipolar disorder, darker winters could alleviate the symptoms of mania, while brighter summers could exacerbate them.