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Human cognitive powers have a seasonal rhythm, and for those living in temperate regions in the northern hemisphere they are strongest in late summer and early autumn. The effect is large enough to tip some older people over the diagnostic threshold for dementia if their cognitive tests are carried out in winter or spring.

Andrew Lim, a neurologist at the Sunnybrook Health Sciences Centre at the University of Toronto, and his colleagues analysed data from 3500 participants aged 60 or over. All of them had undergone tests of their thinking and concentration skills as part of independent studies 鈥� conducted in the US, Canada and France. For some participants, the researchers also looked at levels of proteins associated with Alzheimer鈥檚 disease.

A previous study led by Lim had found a . 鈥淲e wanted to build on this, to see whether this was having a real effect on patients at the clinical level,鈥� says Lim.

Performance peak

The researchers found a strong seasonal trend in both cognitive performance and Alzheimer鈥檚 biomarkers, which was independent of confounding factors such as depression, sleep and physical activity. 鈥淲e found a peak in cognitive performance near the Fall equinox, at the end of summer,鈥� says Lim. The effect was strong enough to make a participant seem nearly five years younger, in terms of age-related cognitive decline, compared to if they were tested in winter or spring.

This finding has important implications, suggesting there is a 30 per cent greater chance of meeting diagnostic criteria for mild cognitive impairment or dementia if cognitive tests are carried out in the winter months. 鈥淭he difference in performance was enough to impact the clinical impression of what diagnostic category a patient was going to be in,鈥� says Lim. It could also impact clinical trial data.

The reasons for the seasonal rhythms are unclear, but the study鈥檚 authors speculate that it could be due to environmental factors such as light and temperature, or physiological factors such as vitamin D or hormone levels. If the biochemical mechanism responsible for the seasonal trend can be pinpointed, this could open the door to new ways of managing dementia, says Lim. 鈥淭he good cognition found in summer could be extended all year round.鈥�

It is also unclear whether the seasonal trend is flipped in the southern hemisphere, as all the study鈥檚 research participants were from the north. 鈥淚t鈥檚 an open question what we would see if we looked at equatorial regions or places in the southern hemisphere. It鈥檚 something we鈥檙e keen to look into,鈥� says Lim.