BIRMINGHAM, Ala. -A cloudy and rainy stretch of summer days can worsen moods and hamper thinking of people who are depressed, according to researchers at the University of Alabama at Birmingham (UAB).

The findings reported in the research journal Environmental Health reveal that impaired mental function can happen beyond winter's dim months, a well-known condition called seasonal affective disorder. Even in summer, a two-week stretch of sunlight deprivation seems to lead the depressed person into a measurable loss of cognitive ability, said Shia Kent, a doctoral student in UAB's School of Public Health and the lead study author.

"The study found an association between decreased exposure to sunlight and increased probability of cognitive impairment," Kent said. The data comes from UAB's Reasons for Geographic and Racial Differences in Stroke (REGARDS) study, which has enrolled thousands of U.S. participants. Kent performed this research for the School of Public Health's Department of Epidemiology.

Comparing a segment of REGARDS participants with no history of stroke to National Aeronautics and Space Administration (NASA) satellite records, the UAB researchers calculated the amount of sunlight that reached the residences of 14,474 study participants. Matching those numbers to specific dates on surveyed cognitive abilities, the researchers found depressed people who were living in dull, cloudy conditions were 36 percent more likely to have cognitive problems compared with those exposed to sunnier skies.

Kent said he and his fellow researchers speculate this is caused by fluctuating levels of two brain hormones, melatonin and serotonin, both of which are impacted by changes in sunlight exposure. Understanding those changes could lead to improved evaluation and treatment guidelines for many disorders, he said.

"These same hormone systems have been implicated in a number of mental disorders and cognitive disorders, such as Alzheimer's, Parkinson's and sleep disorders," Kent said.

Funding came from the National Institute of Neurological Disorders and Stroke, one of the National Institutes of Health.

About the UAB School of Public Health

The School of Public Health is a community of scholars and professionals working and teaching in the different arenas of public health, all with the goal of fostering research, discovery and best-practices crucial to the health of our nation. The school offers more than 20 areas of study and manages dozens of research and community-service centers.