The four seasons are already known to hold some power over our brains, perhaps most famously in the form of seasonal affective disorder.
This is a type of depression that predominantly occurs during the winter months.
Recently, researchers from Sunnybrook Health Sciences Centre and the University of Toronto, both in Canada, set out to see whether Alzheimer's disease might have a seasonal component.
If symptoms do fluctuate across the year, it could be important for both diagnosis and management of the condition. The team's findings are now published in the journal PLOS Medicine.
Seasonal cognitive performance
To investigate whether the seasons do impact dementia, the scientists dipped into data from 3,353 older adults in the United States, Canada, and France. Some participants had been diagnosed with Alzheimer's and others had not received a diagnosis.
All participants went through neuropsychological testing, which included a battery of 19 cognitive tests; also, a subgroup of participants was tested for levels of a protein linked to Alzheimer's.
Once the data had been analyzed, it became clear that average cognitive functioning was better during summer and fall than during winter and spring. The difference was calculated to be the equivalent of 4.8 years of normal cognitive decline.
In line with this, in the winter and spring, the participants were more likely to meet the criteria for dementia or cognitive impairment than in the summer or fall.
In fact, in the colder months, the participants were 31 percent likelier to fall into the range for diagnosis.
As part of their analysis, the researchers controlled for a range of factors that could influence the results, such as depressive symptoms, sleep quality, level of physical activity, and thyroid health. Still, the effect remained statistically significant.
When the scientists compared levels of Alzheimer's-related proteins and gene activity, they found another significant interaction. The levels of relevant proteins and genes found in participants' cerebrospinal fluid fluctuated across the seasons.
If the findings are confirmed, they could be important for the way that dementia is managed. The study authors explain:
"There may be value in increasing dementia-related clinical resources in the winter and early spring when symptoms are likely to be most pronounced."
If seasonality truly does impact dementia, it may also offer up information about how the disease develops; the authors write that "[b]y 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."
As ever, follow-up work will be needed to confirm the results. As the authors explain, the study data did have some limitations; for instance, the scientists only had access to data from people who live in temperate regions of the Northern Hemisphere. Also, each volunteer was only tested once per year.
So, the colder months do appear to worsen symptoms of dementia and reduce cognitive ability in older adults at large.
The new study adds to previous findings published by the same researchers in 2017. In their earlier paper, they uncovered seasonal rhythms of gene expression in the human brain; they also showed that Alzheimer's disease disrupts these rhythms.
This is a fledgling area of research and, due to the length of the annual cycle, it will take a number of lengthy studies before we can finally use the findings to improve dementia diagnosis and care.