There are other possible factors as well. Cold medicines people take in the winter can raise blood pressure, as can non-steroidal anti-inflammatory painkillers.
People also tend to be more depressed in the darker months, leading to more alcohol and coffee consumption, both of which can raise blood pressure, the expert said.
A more obvious reason: People are also often more sedentary in the winter, staying inside and eating more. This can have a secondary effect -- weight gain -- which also contributes to hypertension.
Baker believes that these biological or lifestyle differences are more likely to explain the findings than southern or northern climate or the amount of light.
Overall, however, VA hospitals in all the cities studied showed improvements of about 4 percent per year in their ability to keep patients' average blood pressure under control, the study found.
There's more on lowering blood pressure at the U.S. National Heart, Lung, and Blood Institute.
SOURCES: Kenneth Baker, M.D., professor, internal medicine, Texas A&M Health Science Center College of Medicine; Nov. 5, 2007, presentation, American Heart Association annual meeting, Orlando, Fla.
All rights reserved