Depressed people may prefer the dark but it won't be a good thing for their cognitive abilities, say researchers writing in Environmental Health.

They  used weather data from NASA satellites to measure sunlight exposure across the United States and linked this information to the prevalence of cognitive impairment in depressed people.   Shia Kent, from the University of Alabama at Birmingham, led the team of US researchers who used cross-sectional data from 14,474 people in the NIH-NINDS-funded REGARDS study, a longitudinal study investigating stroke incidence and risk factors, to study associations between depression, cognitive function and sunlight.

"We found that among participants with depression, low exposure to sunlight was associated with a significantly higher predicted probability of cognitive impairment. This relationship remained significant after adjustment for season. This new finding that weather may not only affect mood, but also cognition, has significant implications for the treatment of depression, particularly seasonal affective disorder," said Kent.

They speculate that the physiological mechanisms that give rise to seasonal depression may also be involved in sunlight's effect on cognitive function in the context of depressive symptoms.

Cognitive function was assessed by measurement of short-term recall and temporal orientation. As well as regulating the hormones serotonin and melatonin, light has been shown to also affect brain blood flow, which has in turn been linked with cognitive functions.

The researchers write, "Discovering the environment's impact on cognitive functioning within the context of seasonal disorders may lead not only to better understanding of the disorders, but also to the development of targeted interventions to enhance everyday functioning and quality of life". 

Article: Shia T Kent, Leslie A McClure, William L Crosson, Donna K Arnett, Virginia G Wadley and Nalini Sathiakumar, 'Effect of sunlight exposure on cognitive function among depressed and non-depressed participants: a REGARDS cross-sectional study',  Environmental Health (in press)