Yes, dementia symptoms follow seasonal patterns, and the effect is far larger than most families and clinicians realize. A landmark 2018 study published in PLOS Medicine analyzed 3,353 participants across cohorts in the United States, Canada, and France and found that older adults — both with and without Alzheimer’s disease — scored significantly better on cognitive tests in late summer and early fall than in winter and spring. The seasonal gap was equivalent to roughly 4.8 years of age-related cognitive decline, meaning a person tested in February could appear nearly five years more impaired than the same person tested in August.
This finding has real consequences for anyone caring for a loved one with dementia or navigating a diagnosis. If your parent’s cognitive evaluation happened in January, the results may paint a darker picture than what you would see in September. And if you have noticed that confusion, agitation, or memory lapses seem to worsen as the days get shorter, you are not imagining it — there is a biological basis for what you are observing. This article covers the research behind seasonal cognitive fluctuations, the biological mechanisms driving them, how circadian rhythm disruption compounds the problem, why sundowning gets worse in colder months, and what interventions like light therapy may offer.
Table of Contents
- How Do Seasonal Patterns Affect Dementia Progression and Cognitive Function?
- What Biological Mechanisms Drive Seasonal Cognitive Changes in Dementia?
- How Circadian Rhythm Disruption Increases Dementia Risk
- Managing Sundowning When Seasonal Changes Make It Worse
- What Light Therapy Can and Cannot Do for Dementia Patients
- Why the Timing of Cognitive Assessments Matters
- Where Seasonal Dementia Research Is Heading
- Conclusion
- Frequently Asked Questions
How Do Seasonal Patterns Affect Dementia Progression and Cognitive Function?
The PLOS Medicine study remains the most cited research on this question, and its findings are striking in their consistency. Cognitive performance peaked just before the fall equinox and dropped to its lowest point near the spring equinox, with the most pronounced effects on working memory and perceptual speed. This was not a marginal statistical blip. Across three independent cohorts spanning different countries and demographics, the pattern held. Researchers at Columbia University Irving Medical Center noted that the seasonal rhythm existed in cognitively healthy older adults as well, suggesting this is a feature of aging brains generally — one that becomes clinically significant when layered on top of neurodegenerative disease. Perhaps most troubling, a diagnosis of mild cognitive impairment or Alzheimer’s disease was more likely to occur during colder months.
As Alzheimer’s Research UK reported, this means the calendar date of an assessment can influence whether someone crosses the threshold for a clinical diagnosis. Consider two people with identical underlying pathology: one tested in July, the other in March. The March patient is more likely to receive a formal diagnosis, which can trigger changes in medication, legal planning, and care arrangements. This is not a flaw in the diagnostic criteria so much as a reminder that cognition is not static — it fluctuates with the seasons, and our clinical systems have not fully accounted for that variability. Researchers from the study suggested there may be value in increasing dementia-related clinical resources in winter and early spring, when symptoms are most pronounced and families are most likely to seek help. ALZFORUM analysts echoed this concern, noting that clinical assessments conducted in winter may overestimate cognitive impairment compared to those done in summer.
What Biological Mechanisms Drive Seasonal Cognitive Changes in Dementia?
The seasonal swings are not simply a matter of mood or motivation. The PLOS Medicine research identified measurable biological changes tied to the calendar. Cerebrospinal fluid levels of amyloid-beta 42, a protein central to Alzheimer’s pathology, peaked in summer months. Additionally, the researchers found seasonal rhythms in brain gene expression involving four cognition-associated gene modules linked to transcription factors including BCL11A, CTCF, EGR1, MEF2C, and THAP1. These are not obscure laboratory curiosities — MEF2C, for instance, plays a well-established role in synaptic plasticity and has been implicated in intellectual disability and neurodegeneration. On a more accessible level, two neurotransmitter systems shift with the seasons in ways that directly affect dementia patients. Melatonin production increases as daylight hours shrink in fall and winter, contributing to daytime grogginess and fragmented sleep.
Meanwhile, serotonin levels drop with reduced sunlight exposure, which can worsen depression and anxiety — conditions already common in people with dementia. The compounding effect matters here. A person whose brain is already struggling to clear amyloid plaques during sleep now sleeps worse because of excess melatonin, and their mood deteriorates because serotonin is in short supply. Each factor amplifies the others. However, it is worth noting a limitation: most of this research has been conducted in temperate climates with pronounced seasonal daylight variation. Whether the same patterns hold near the equator, where daylight hours change minimally across the year, remains an open question. Families living in tropical regions should not assume these findings apply uniformly to their situation.
How Circadian Rhythm Disruption Increases Dementia Risk
A study published on December 29, 2025, in the journal Neurology added an important dimension to the seasonal picture. Researchers followed 2,183 people with an average age of 79, none of whom had dementia at the start of the study, for an average of three years. During that period, 176 participants developed dementia. Those with the weakest circadian rhythms — measured through wrist-worn activity monitors — had nearly 2.5 times the risk of developing dementia compared to those with robust daily activity patterns. The timing of daily activity mattered independently of its intensity. People whose peak daily activity occurred after 2:15 PM had a 45 percent increased risk of dementia compared to those who peaked between 1:11 and 2:14 PM.
This is a surprisingly narrow window, and it suggests that even subtle shifts in the body’s internal clock may reflect or contribute to neurodegeneration. The proposed mechanism is that disrupted circadian rhythms increase systemic inflammation and interfere with the brain’s ability to clear amyloid proteins during sleep — a process sometimes called the glymphatic system. When deep sleep is fragmented or poorly timed, amyloid waste accumulates rather than being flushed out. For caregivers, this research offers a practical signal. If a loved one begins sleeping later, staying inactive through the morning, and not hitting their stride until mid-afternoon, that shift may be more than a lifestyle change. It could be an early marker of cognitive decline worth discussing with a physician.
Managing Sundowning When Seasonal Changes Make It Worse
Sundowning — the pattern of increased confusion, agitation, pacing, or emotional distress in the late afternoon and evening — is one of the most distressing aspects of dementia care. Estimates of its prevalence vary enormously, ranging from 1.6 percent to 66 percent of dementia patients depending on the study and the care setting. That wide range reflects differences in how sundowning is defined and measured, but virtually every dementia caregiver recognizes the phenomenon. Sundowning worsens in fall and winter for a straightforward reason: the sun sets earlier, and the transition from light to dark — a key trigger — happens during the most active hours of the day. The overlap between sundowning and seasonal affective disorder compounds the problem.
About 5 percent of American adults experience SAD, and its symptoms of low energy, social withdrawal, and irritability are more severe in people who also have dementia-related sundowning. A person dealing with both conditions faces a double burden that neither condition alone would predict. The tradeoff caregivers face is real. Keeping a person with dementia active and engaged during daylight hours can help anchor their circadian rhythm and reduce sundowning severity, but pushing too hard risks overstimulation, which can itself trigger agitation. The goal is consistent, gentle routine — meals at the same time, walks during morning light, calm activities as evening approaches — rather than dramatic interventions. Blackout curtains and bright evening lighting can also blur the abrupt light-to-dark transition that often sets off sundowning episodes.
What Light Therapy Can and Cannot Do for Dementia Patients
Light therapy has emerged as one of the more promising non-pharmacological interventions for seasonal cognitive decline in dementia. A systematic review and meta-analysis of 12 studies involving 766 patients found that phototherapy improved cognitive function as measured by the Mini-Mental State Examination. The Alzheimer’s Society UK notes that bright light therapy helps regulate the circadian rhythm in dementia patients, improving sleep patterns and potentially boosting mood, memory retention, and reasoning. A more experimental approach involves 40 Hz flickering light therapy, which is being studied for its potential to promote gamma-wave brain activity and activate microglia — the brain’s immune cells — that clear beta-amyloid plaques. This research, referenced on Alzheimers.gov, is still in clinical trial stages and should not be treated as an established treatment.
The distinction matters because consumer devices claiming to deliver 40 Hz light stimulation are already on the market, and their efficacy and safety outside of controlled research settings have not been established. A key limitation of light therapy generally is compliance. Many dementia patients will not sit in front of a light box for 30 to 60 minutes, particularly if they are in later stages of the disease. Caregivers sometimes integrate bright lighting into living spaces rather than relying on dedicated sessions, but the optimal intensity, timing, and duration of exposure are still being refined. Light therapy is best understood as a helpful supplement to broader circadian and seasonal management strategies, not a standalone solution.
Why the Timing of Cognitive Assessments Matters
The seasonal variability in cognition creates a practical problem that many families never consider: when your loved one gets tested can shape the diagnosis they receive. A person evaluated in February during the seasonal trough may meet criteria for mild cognitive impairment, while the same person tested in August might fall within normal range. This does not mean the winter result is wrong — the impairment is real in that moment — but it means that a single assessment provides an incomplete picture.
Clinicians who are aware of this research can account for it by noting the season of testing, comparing results across multiple time points, or adjusting their interpretation of borderline scores. Families can advocate for this kind of nuance, particularly if a diagnosis feels inconsistent with what they observe at home during other times of year. Requesting a follow-up assessment six months later is reasonable and can provide a more accurate baseline.
Where Seasonal Dementia Research Is Heading
The convergence of seasonal cognition research, circadian rhythm studies, and light therapy trials points toward a future where dementia care becomes more time-aware. Rather than treating cognitive decline as a steady, linear process, clinicians may begin to account for seasonal rhythms in treatment planning, assessment scheduling, and intervention timing. The December 2025 Neurology study on circadian disruption, for example, suggests that wearable activity monitors could eventually serve as early warning systems for dementia risk — flagging subtle shifts in daily rhythms before cognitive symptoms become obvious.
The gene expression findings from the PLOS Medicine study open a longer-term research avenue. If specific transcription factors like EGR1 and MEF2C drive seasonal cognitive variation, therapies that stabilize their expression across seasons could theoretically reduce the winter trough. That work is years away from clinical application, but it reframes seasonal cognitive decline as a potentially modifiable biological process rather than an inevitable feature of aging.
Conclusion
Seasonal patterns in dementia are not folklore or caregiver bias. Large, well-designed studies confirm that cognition peaks in late summer and early fall and declines significantly in winter and spring, driven by measurable changes in brain gene expression, amyloid protein levels, melatonin, and serotonin. Circadian rhythm disruption independently raises dementia risk, and sundowning worsens as daylight hours shrink. These patterns mean that the timing of clinical assessments, the structure of daily routines, and the use of interventions like light therapy all matter more than most care plans currently acknowledge.
For families and caregivers, the practical takeaways are straightforward. Be aware that winter months may bring a noticeable decline that partially reverses in summer. Advocate for repeat cognitive assessments across different seasons if a diagnosis feels uncertain. Prioritize morning light exposure, consistent daily routines, and calm evening environments to support circadian stability. And discuss light therapy options with a physician, recognizing that while the evidence is encouraging, it remains a complement to — not a replacement for — comprehensive dementia care.
Frequently Asked Questions
Is the seasonal decline in dementia symptoms permanent, or does cognition recover in summer?
Research shows the pattern is cyclical. Cognitive performance tends to improve in late summer and early fall and decline again in winter and spring. However, the underlying disease continues to progress, so each year’s summer peak may be somewhat lower than the previous year’s.
Can seasonal cognitive changes cause a misdiagnosis of dementia?
Not exactly a misdiagnosis, but the timing of testing can influence results. Clinical assessments conducted in winter may overestimate cognitive impairment compared to summer assessments. A person tested during the seasonal trough may meet diagnostic thresholds they would not meet at other times of year.
Does light therapy actually slow the progression of Alzheimer’s disease?
A meta-analysis of 12 studies found that phototherapy improved cognitive scores on the MMSE, but there is not yet strong evidence that it slows the underlying disease progression. The 40 Hz flickering light therapy approach is still in clinical trials. Light therapy is best viewed as a tool for managing symptoms, particularly sleep disruption and mood changes.
How much does circadian rhythm disruption increase dementia risk?
According to a 2025 study in Neurology, people with the weakest circadian rhythms had nearly 2.5 times the risk of developing dementia compared to those with strong rhythms. Those whose daily activity peaked after 2:15 PM had a 45 percent increased risk.
Are seasonal patterns in dementia the same everywhere in the world?
Most of the research has been conducted in temperate climates with significant seasonal daylight variation. Whether the same patterns hold in tropical regions near the equator, where daylight hours change little across the year, has not been well studied.
What can caregivers do during winter months to help manage seasonal worsening?
Maintain consistent daily routines, maximize morning light exposure through walks or bright indoor lighting, keep evening environments calm and well-lit to ease the light-to-dark transition, and discuss bright light therapy with a healthcare provider. Monitoring for increased sundowning symptoms and adjusting activities accordingly is also important.
