Specifically, the study found that older people who took cognitive tests during winter and early spring tended to have lower scores — particularly for tasks involving working memory and perceptual speed — than those who took the tests at other times of the year.
The study also found that when older adults undergo cognitive testing in winter and spring, they are significantly more likely to meet the diagnosis of mild cognitive impairment than if they took the tests in summer or early fall. Mild cognitive impairment is a controversial condition that has been suggested as a possible precursor to the development of dementia.
“The seasonal difference is significant,” said Dr. Philip De Jager, the study’s senior author and a neurologist at Columbia University, in a released statement. “Cognitive function normally declines with age, and the seasonal difference we found is roughly equivalent to four years of aging. This means that an older person functioned as if they were four years older in the winter, compared to how they functioned in the summer.”
For the study, De Jager and his colleagues analyzed data collected from a string of cognitive tests conducted on 2,761 healthy older adults and 592 dementia patients. The patients came from different racial and ethnic groups living in the United States, Canada and France. Most were in their 70s or older.
The participants underwent the testing on a single day, but those days were scattered throughout the year. When the researchers compared the testing dates with the results, they found that people tested in July through October scored better, on average, than those tested in the other months.
Indeed, the researchers found that scores tended to peak around the fall equinox in late September.
These findings held true for both the healthy adults and those diagnosed with Alzheimer’s disease, although the seasonal differences were smaller among those with dementia.
The two cognitive skills most affected by the seasons were working memory (the ability to hold information in the mind for a short time, such as while memorizing a pin number) and perceptual speed (the time it takes to recognize or compare symbols or figures or to do other simple tasks).
The seasonal association was so “robust,” say the researchers, that the participants’ scores were 30 percent more likely to meet the diagnostic criteria for mild cognitive impairment when the tests were taken in the winter and spring than in the summer and fall.
Overall, the average decline in cognitive functioning observed during winter and early spring was about the same as if the participants had aged by 4.8 years, the researchers concluded.
These findings held even after adjusting for possible confounding factors, such as the time of day the test was given and the participants’ mood, self-reported hours of sleep and physical activity, and thyroid health. (Some research suggests that levels of thyroid hormones can affect cognition even in people who have not been diagnosed with a thyroid condition.)
The researchers also examined the spinal fluid taken from some of the participants in the study who were diagnosed with dementia, as well as the autopsied brains of participants who had died.
In those samples they found more evidence of seasonal rhythms, specifically in the levels of Alzheimer-disease-related proteins in the spinal fluid and in the expression of dementia-related genes in the brain.
What might be driving these seasonal effects is unclear. It may be environmental factors, such as light and temperature, De Jager and his co-authors write. Or it could be cognition-impacting factors not adjusted for in the study, such as older people being more socially isolated during the winter months or having seasonal-related injuries or pain. (The symptoms of rheumatoid arthritis, for example, tend to worsen when weather turns cold and damp).
Limitations and implications
The study comes with important caveats. To begin with, it is an observational study, so it can show only a correlation between cognitive function and the changing seasons, not a cause-and-effect relationship. Furthermore, the participants were tested only once. They weren’t repeatedly evaluated throughout the year to determine if their cognitive skills changed with the seasons.
Another limitation is that the study involved people living only in temperate climates of the Northern Hemisphere. The results might have been different if it had involved individuals from warmer climates with less seasonal variation.
Still, this study’s key finding — that season may be associated with cognitive function — is not that far-fetched. As background information provided in the paper points out, we already know that seasonal rhythms modulate human behavior and physiology, including brain functions. People with seasonal affective disorder (SAD), for example, experience a distinct drop in their mood during late fall and winter. Research has also found that the onset of schizophrenia symptoms is most likely to occur in late summer.
As Rosa Sancho, head of research at the British charity Alzheimer’s Research UK, told the Daily Mail, the current study suggests “that researchers may need to take the time of year into account when measuring how diseases like Alzheimer’s develop over time, and there may be a need for additional care and support provision in the winter months but we need to see more work to be sure of these findings.”
But for individuals, the implications of the study’s findings are less clear.
‘Dementia is complex, caused by a mix of age, genetics, and lifestyle factors, and the results of this study don’t mean we should all rush to book sunshine breaks over the winter months,” said Sancho.
Yet, “while there is no sure-fire way to prevent dementia, research has highlighted things we can do to reduce our risk of the condition,” she added. “The best current advice to keep our brain healthy as we age is to exercise regularly, eat a balanced diet, maintain a healthy weight, not smoke, and keep blood pressure and cholesterol in check.”
FMI: The study can be read in full online at the PLOS Medicine website.