A lack of sunshine may cloud memory and other thinking or "cognitive" functions in some people with depression, a new study hints.
There is a well-known association between sunlight exposure and mood, the clearest example being seasonal affective disorder (SAD) — a form of depression in which symptoms shift with the seasons, usually arising in the late fall and winter and improving in sunnier months.
But little is known about whether sunshine can affect thinking and memory.
For the new study, researchers at the University of Alabama at Birmingham looked at the correlation between NASA weather data and cognitive-test scores among more than 14,000 U.S. adults age 45 and older. All had taken part in a government study of stroke risk factors.
The researchers found that among people who had screened positive for depression, those who had been exposed to little sunshine over a two-week period tended to have lower cognitive scores than their counterparts who lived in sunnier climes.
Depressed adults from the least sunny areas were more than twice as likely to have impairments in memory and other cognitive functions as those with the greatest sun exposure.
The sunshine-cognition link was not seen in adults without depression, however.
The findings, published in the journal Environmental Health, do not prove that a lack of sun impairs depressed people's thinking — or that basking in the sun will improve the situation.
"You obviously don't want to take any actions based on one study," lead researcher Shia T. Kent, a Ph.D. candidate at the Alabama university, told Reuters Health.
But in theory, he said, sunlight might affect cognition through the same pathways it is thought to impact mood. Sun exposure helps regulate levels of two hormones, melatonin and serotonin, that affect mood and are suspected of playing a role in SAD and general depression.
Recent research suggests that melatonin and serotonin are also involved in cognition.
"It may be that people who are more affected by sunlight exposure in terms of depression are also more affected in terms of cognition," Kent speculated.
The findings also raise the possibility that light therapy, which is a standard treatment for SAD, might improve depressed individuals' cognition as well.
Kent said that future studies of SAD patients, including those testing light therapy, should look at any potential effects on cognition.