Impact of sunlight on the age of onset of bipolar disorder

Abstract

Objective: Although bipolar disorder has high heritability, the onset occurs during several decades of life, suggesting that social and environmental factors may have considerable influence on disease onset. This study examined the association between the age of onset and sunlight at the location of onset.

Method: Data were obtained from 2414 patients with a diagnosis of bipolar I disorder, according to DSM-IV criteria. Data were collected at 24 sites in 13 countries spanning latitudes 6.3 to 63.4 degrees from the equator, including data from both hemispheres. The age of onset and location of onset were obtained retrospectively, from patient records and/or direct interviews. Solar insolation data, or the amount of electromagnetic energy striking the surface of the earth, were obtained from the NASA Surface Meteorology and Solar Energy (SSE) database for each location of onset.

Results: The larger the maximum monthly increase in solar insolation at the location of onset, the younger the age of onset (coefficient= -4.724, 95% CI: -8.124 to -1.323, p=0.006), controlling for each country's median age. The maximum monthly increase in solar insolation occurred in springtime. No relationships were found between the age of onset and latitude, yearly total solar insolation, and the maximum monthly decrease in solar insolation. The largest maximum monthly increases in solar insolation occurred in diverse environments, including Norway, arid areas in California, and Chile.

Conclusion: The large maximum monthly increase in sunlight in springtime may have an important influence on the onset of bipolar disorder.

Fig. 1

Monthly solar insolation patterns at northern, temperate, and equatorial latitudes. The pattern of monthly solar insolation for three sites in this study: Trondheim, Norway (latitude 64.0 N), Kansas City, MO, USA (latitude 39.1 degrees N) and Medellin, Colombia (latitude 6.3 degrees N).

Fig. 2

Monthly solar insolation values for cities at the same latitude. Locations at the same latitude may have very different solar insolation due to local conditions. The latitude of Nice, France, is 43.70 degrees N, with a maximum monthly increase in solar insolation of 1.31 kWh/m²/day between February and March. The latitude of Toronto, Canada is 43.67 degrees N, with a maximum monthly increase in solar insolation of 1.04 kWh/m²/day between March and April.