(April 23, 2019) For individuals with bipolar I disorder, the more severe form of the illness, a drastic change in sunlight between winter and summer may be associated with increased suicide attempts, according to new international research.
Research has long suggested a link between the natural environment and symptoms of mental illness. Seasonal affective disorder, which affects an estimated 10 million Americans, is characterized as depression related to seasonal changes, and most often occurs in fall and winter when sunlight is scarce. A similar pattern has been shown for some individuals with bipolar disorder—when mania, or a period of excessive euphoria or excitement, tends to occur during spring or summer months, and depression tends to come about in the fall and winter. And while studies on the topic draw varying conclusions, a majority of research on seasonal changes in suicide rates finds a somewhat paradoxical relationship, indicating increased rates in the spring and summer.
Recently, an international team of researchers conducted an exploratory study to further investigate the link between seasonal changes in sunlight and suicide attempts of individuals with bipolar I disorder. The researchers studied available data for 3,365 individuals diagnosed with bipolar I disorder between 2010 and 2016. Data included limited demographic information, history of suicide attempts, birth location, onset location of bipolar disorder, and current location. The study population included people with 310 onset locations in 51 countries. A majority of individuals were living in the same country and city in which onset occurred.
The researchers then calculated the average solar insolation—the amount of the sun’s electromagnetic energy hitting Earth’s surface—for each onset location during winter and summer months. For locations in the southern hemisphere, data were shifted by 6 months to approximate similar seasons in the northern hemisphere. The difference between the amount of sunlight observed during the winter and the amount observed during the summer was greatest near the Earth’s poles, according to the results. For locations closer to the equator, levels of sunlight remain much more constant in the transition from winter to summer.
The study authors highlight previous research that offers context for the findings. As humans have adapted to 24-hour light-dark cycles, regulation of physiological processes depends on maintaining alignment with one’s environment using external cues such as available sunlight. Certain neurotransmitters, such as serotonin, and Vitamin D have also been shown to be lowest during the winter. Both of these biological molecules affect mood regulation, and deficiencies have been linked with depression and suicidal behavior.When applying these findings to the study population’s suicide data, the researchers found that the risk of attempting suicide increased significantly when the location where a person’s bipolar disorder first manifested saw more extreme changes in sunlight throughout the year. While 31.1% of all studied individuals had a history of attempted suicide, the odds of an attempt increased by 49% for individuals with onset locations closest to the North Pole, compared to locations closest to the equator.
The authors note that the study comes with many limitations. In particular, data were not available for individual characteristics likely associated with risk of suicide, such as socioeconomic indicators, other psychiatric or medical conditions, or psychiatric treatment history. However, the findings provide further evidence of the impact of sunlight on the expression of mental illness, meriting additional exploration of the topic.
References:
Jessica Walthall
Research and Advocacy Associate
Treatment Advocacy Center