We’re used to seasonal weather changes, but now, research suggests that older adults’ brains also go through seasonal changes—changes that make their brains act five years older, meaning five years slower, at certain times of year.

Researchers at University of Toronto analyzed health data on more than 3,000 people, age 70 to 90, from the US, Canada and France. Most of these people were cognitively healthy, but some of them had been diagnosed with Alzheimer’s disease.

Results: On average, the ability to think, concentrate and remember was better in the late summer and early fall than in the winter and spring. That held true independently of whether the participant had Alzheimer’s. Lab tests also showed that among the participants with Alzheimer’s, genes and proteins linked to that disease followed a seasonal pattern. And when participants were tested for early cognitive impairment, there was a 30% higher chance of meeting the diagnostic criteria in winter or spring. The difference in cognition between winter-spring and summer-fall was the equivalent of almost five years of brain age. The researchers dubbed this effect seasonal plasticity of cognition.

While the study didn’t answer why seasonal plasticity of cognition occurs, the researchers speculated that several factors may be at work. With longer days and warmer temperatures, people may be more active and sleep better, which may improve cognition. Another possibility is the effect season may have on hormones that affect cognition, such as testosterone (even in women) and melatonin. Finally, humans may have an internal clock—akin to a circadian rhythm—embedded in their brains through evolution that drives cognition. Why might that be? Circadian rhythms and seasonal affective disorder (SAD) respond to light treatment, demonstrating that light stimulates our brains. And in fact, the study found that peak cognition occurred during the fall equinox—the time of year when participants would have had the most cumulative exposure to light.

What’s this all mean for you? For one thing, if you’ve always suspected that you’re just not as sharp mentally during the colder (darker) months, this study shows that you could well be correct and that this phenomenon is normal. It also means it’s reasonable to think that being sure to expose yourself to sunlight from late fall through winter might not only make you happier at those times—it might help you think better. And, the researchers suggest, for older people with known cognitive decline, additional services in the winter and spring—when cumulative sunlight exposure is lowest—might be of great benefit. The researchers had another suggestion, too: Health-care providers who suspect a patient has cognitive impairment might test in winter or spring to get an earlier diagnosis so that treatment can start earlier as well.