The findings showed that average cognitive functioning was higher in the summer and fall than the winter and spring, equivalent in cognitive effect to 4.8 years difference in age-related decline.
In addition, the odds of meeting the diagnostic criteria for mild cognitive impairment or dementia were higher in the winter and spring than summer or fall.
The association with seasonality was also seen in levels of Alzheimer's-related proteins and genes in cerebrospinal fluid and the brain, the researchers said.
"There may be value in increasing dementia-related clinical resources in the winter and early spring when symptoms are likely to be most pronounced," said the researchers led by Andrew Lim from the University of Toronto in Canada.
"By shedding light on the mechanisms underlying the seasonal improvement in cognition in the summer and early fall, these findings also open the door to new avenues of treatment for Alzheimer's disease," Lim said.
There have been few previous studies concerning the association between season and cognition in older adults.
For the new study, published in the journal PLOS Medicine, the team analysed data on 3,353 people.
Participants had undergone neuropsychological testing and, for some participants, levels of proteins and genes associated with Alzheimer's disease were available.
The association between season and cognitive function remained significant even when the data was controlled for potential confounders, including depression, sleep, physical activity, and thyroid status, the researchers noted.
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