Patients with bipolar disorder who live in places with large changes in solar insolation between winter and summer may be at higher risk for suicide attempts, according to a study published in the Journal of Psychiatric Research. Solar insolation is the amount of the sun’s electromagnetic energy striking a surface area on the earth.

Between 2010 and 2016, investigators analyzed data from 3365 patients with bipolar I disorder from 51 countries. They collected all patients’ suicide-related data, birth place, onset location of bipolar disorder, and current location. Using the NASA POWER database, investigators collected monthly solar insolation averages for the locations and calculated the ratio of the mean northern hemisphere winter (December, January, February) to the mean summer (June, July, August) insolation. Southern hemisphere insolation data were shifted by 6 months to account for the seasonal cycle.

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Approximately 31% of the patients had a history of suicide attempt. Analyses revealed that patients from locations with larger differences between mean winter and summer insolation (small ratio) had more suicide attempts, whereas those who resided in locations with smaller differences (high ratio) had fewer suicide attempts. When investigators compared the largest ratio (near the equator) with the smallest ratio (near the North Pole), they found a 49% decrease in the odds of suicide attempt.

Being female, having a history of alcohol or substance abuse, and being in a younger birth cohort were positively associated with suicide attempt, and living in a country with a state-sponsored religion decreased the association (P <.01 for all).

No data were available to investigators regarding individual parameters associated with suicide risk. There were also no data regarding whether patients received long-term lithium treatment or other therapies that target or modify the circadian system.

“Given the increased recognition of the importance of sunlight on human behavior and the frequent presence of circadian rhythm dysfunction in bipolar disorder, more knowledge of the relation of solar insolation to suicide attempts is needed,” investigators stated.

Reference

Bauer M, Glenn T, Alda M, et al. Association between solar insolation and a history of suicide attempts in bipolar I disorder. J Psychiatr Res. 2019;113:1-9.