Nature and nurture in Parkinson’s disease
Parkinson’s disease is the fastest-growing neurological condition in the world. Current projections suggest that the number of people living with the disease will more than double — from around 11.8 million cases in 2021 to over 25 million by 2050 (Journal of Neural Transmission, 2025) – Environmental toxicants and Parkinson’s disease: recent evidence, risks, and prevention opportunities.
Against that backdrop, a growing body of research is pointing to something deceptively simple: one of the most accessible tools for reducing risk and supporting quality of life may be the light above us.
This article explores what we now understand about the genetics and environment of Parkinson’s, the evidence linking light to the disease’s biology, and practical steps that anyone can take today.
Genes, Environment, and the Space Between
Most of us think of Parkinson’s as a genetic condition — something that ran in families, driven by inherited mutations.
We now understand that the reality is far more nuanced, and for most people, far more hopeful.
The heritability of Parkinson’s disease — the proportion of cases attributable to genetic factors — is currently estimated at between 22% and 40% – The epidemiology of Parkinson’s disease. Only around 5–15% of cases are caused by clearly inherited, single-gene mutations, and the Parkinson’s Foundation estimates that fewer than 13% of people with the condition have an identifiable genetic link. For the vast majority — the remaining 85–90% — the cause involves a complex interplay between genetic predisposition, ageing, and the environment.
This matters, because it means that most of the disease burden is at least potentially modifiable. A landmark 2025 review in The Lancet Neurology. concluded that large-scale genetic studies have confirmed environmental factors — rather than genetic predisposition — play a dominant role in causing Parkinson’s for most people. The three categories of environmental exposure for which the strongest evidence exists are certain pesticides, the solvents trichloroethylene (TCE) and perchloroethylene (PCE), and air pollution.
But here is the important corollary: just as harmful environmental exposures can increase risk, protective environmental exposures — including light — may reduce it.
A large longitudinal study published in Frontiers in Human Neuroscience (2025) – The global rise in Parkinson’s disease: a critical analysis of causes and future directions, using UK Biobank data from nearly 375,000 adults, estimated that effectively targeting modifiable risk factors could potentially prevent more than 33% of Parkinson’s cases in the current population. Time spent outdoors in sunlight was among the factors significantly associated with reduced risk.
Light and the Parkinson’s Brain: What We Know
The evidence connecting light exposure to Parkinson’s disease risk and progression operates through several distinct biological pathways. Understanding these helps explain why light is not simply a ‘nice to have’ but a genuine neuroprotective factor.
- Vitamin D and dopaminergic neuroprotection
The most widely understood mechanism is vitamin D synthesis. Sunlight — specifically ultraviolet-B radiation — triggers the skin to produce vitamin D3, and the Parkinson’s-relevant biology here is striking. Vitamin D receptors and the enzyme that activates vitamin D (1α-hydroxylase) are found in particularly high concentrations in the substantia nigra — the precise region of the brain where dopaminergic neurons are lost in Parkinson’s disease. Vitamin D plays roles in neurotrophic regulation, antioxidant defence, and neuroprotection, and multiple studies have found that people with Parkinson’s tend to have lower circulating vitamin D levels than age-matched controls.A large prospective cohort study published in BMC Neurology (2024) –Association of time spent outdoors with the risk of Parkinson’s disease: a prospective cohort study of 329,359 participants, drawing on 329,359 UK Biobank participants, found that those spending more time outdoors had a significantly lower risk of developing Parkinson’s, and that higher serum vitamin D levels helped mediate this protective association.
- Sunlight and the dopamine transporter
In a striking 2023 study published in Movement Disorders, researchers at Amsterdam UMC analysed brain scan data from 730 participants in the Parkinson’s Progression Marker Initiative database – Dopamine Transporter Availability in Early Parkinson’s Disease is Dependent on Sunlight Exposure. They found that in people with early Parkinson’s disease, the availability of dopamine transporters in the brain was measurably higher during spring and summer, and was positively associated with greater sunlight exposure in the preceding month. The implication — carefully framed as preliminary — is that daylight may directly influence the dopaminergic system, not just through vitamin D but through other biological mechanisms that remain under investigation. - Circadian disruption as a disease pathway
Light is the primary synchroniser of the body’s circadian clock, governed by the suprachiasmatic nucleus in the hypothalamus. In Parkinson’s disease, this clock goes awry: the disease disrupts circadian clock gene expression, and the resulting circadian dysregulation — with a flattened, phase-advanced rhythm — compounds sleep disturbance, depression, and very likely accelerates neurodegeneration. A comprehensive 2025 review in npj Parkinson’s Disease concluded that circadian disruptions in Parkinson’s not only worsen symptoms but may accelerate disease progression by influencing mitochondrial function, oxidative stress, and neuroinflammation.
Regular outdoor light exposure is one of the most powerful tools available to anchor and strengthen circadian rhythms — particularly morning sunlight, which resets the clock for the day – Circadian clock dysfunction in Parkinson’s disease: mechanisms, consequences, and therapeutic strategy. - The outdoor time effect — and genetics
A particularly important finding comes from a 2025 study in the Journal of Health, Population and Nutrition, which examined outdoor light exposure, genetic risk, and PD incidence in 375,599 UK Biobank participants – Outdoor light spending time, genetic predisposition and incident Parkinson’s disease: the mediating effect of lifestyle and vitamin D. Those spending more time outdoors in summer had a 23% lower risk of developing Parkinson’s (HR 0.77; 95% CI 0.68–0.88). Crucially, the protective effect was most pronounced in people with high genetic risk — suggesting that for those who carry a genetic predisposition, outdoor light exposure may be one of the most important modifiable factors available to them. The mechanisms identified included physical activity, sleep patterns, and vitamin D — with physical activity alone mediating around 16% of the protective effect.
Managing Progression: The Role of Light Therapy
For people already living with Parkinson’s, the relevance of light doesn’t end with prevention. A growing body of clinical evidence suggests that light therapy can meaningfully improve some of the most disabling non-motor features of the condition — particularly sleep, mood, and daily wellbeing.
Sleep disorders affect the majority of people with Parkinson’s, and standard pharmacological treatments often have limited effectiveness and significant side effects. A randomised controlled trial published in eClinicalMedicine (The Lancet) in 2024 – Efficacy of biologically-directed daylight therapy on sleep and circadian rhythm in Parkinson’s disease: a randomised, double-blind, parallel-group, active-controlled, phase 2 clinical trial, conducted at Queensland University of Technology, tested a personalised bright light therapy intervention in people with mild to moderate Parkinson’s disease. The intervention — tailored to each individual’s circadian chronotype — significantly improved restorative deep sleep, affect, and overall wellbeing, with benefits sustained two weeks after treatment ended. Caregivers also reported improvements in their partners’ night-time restlessness, a finding with real significance for quality of life across the household.
A further multi-centre randomised controlled trial published in Neurotherapeutics (2025) found that daily circadian-effective light therapy — using a spectrum specifically targeting the circadian system — produced improvements in both motor and non-motor symptoms as assessed by the MDS-UPDRS rating scale, with significant improvement in quality of life measures and good tolerability over six months – A double-blind, controlled trial of circadian effective light therapy in patients with Parkinson’s disease.
A meta-analysis of five RCTs found that light therapy was associated with significant improvements in motor function, depression, and sleep disturbance in people with Parkinson’s — a consistent signal across multiple independent research groups.
Practical Tips: What the Evidence Supports
The science is clear enough to translate into actionable, everyday habits. None of these demand special equipment or cost the earth. All are supported by published research from the past three years.
☀️ Prioritise morning outdoor time Morning sunlight is the most biologically potent for circadian anchoring. Even 20–30 minutes outdoors in the morning — for a walk, a coffee in the garden, or simply sitting by a window in bright conditions — can meaningfully support the body’s daily rhythm. For people with Parkinson’s, this doubles as an opportunity for physical activity, which independently reduces disease risk and is one of the most robustly evidenced interventions for slowing progression.
☀️ Aim for consistent outdoor time across the week The UK Biobank studies found a dose-dependent relationship between outdoor time and reduced Parkinson’s risk. Consistent daily exposure — not just weekend bursts — provides the most benefit. Even in winter months, outdoor light exposure (even on overcast days) provides meaningful circadian input that indoor lighting cannot replicate.
☀️ Consider your vitamin D status People at higher latitudes, those who spend most of their time indoors, and those with darker skin tones who live in northern climates are at particular risk of vitamin D deficiency. For those with Parkinson’s disease or a family history that raises concern, discussing vitamin D levels and supplementation with a GP is worthwhile. The evidence for dietary sources alone (oily fish, eggs, fortified foods) is helpful but insufficient at UK latitudes through winter.
🌙 Manage evening light to protect sleep This is the less-discussed side of the equation. Evening exposure to bright, blue-spectrum light — from overhead lighting, screens, and devices — suppresses melatonin production and disrupts the circadian wind-down that precedes healthy sleep. For people with Parkinson’s, whose sleep architecture is already compromised, managing evening light is particularly important. Warm, dim lighting after 8pm, and reducing screen use in the hour before bed, support the biological conditions for better sleep.
💡 For those already living with Parkinson’s: discuss light therapy Bright light therapy — using a clinical lightbox delivering 10,000 lux for 20–30 minutes in the morning — has a growing evidence base for sleep improvement, mood, and potentially motor function in Parkinson’s. It is non-pharmacological, well-tolerated, and low-risk. Bringing this conversation to a GP or neurologist, particularly if sleep disturbance is a significant challenge, is increasingly well-supported by the research.
🏠 Think about the indoor lighting environment. With the best will in the world, most of us will spend most of our lives indoors. Move furniture close to the window and boost with bright, cool artificial lighting during the day, switching to softer, warmer light at night and installing blackout curtains for complete darkness at night can help to keep the body clock on track.
Taking a balanced view
It is important to be honest about where the science stands. The evidence linking outdoor light exposure to reduced Parkinson’s risk is substantial and growing, especially through the contribution of large-scale biobank studies. But not yet proven through randomised controlled trials, so we cannot yet say with certainty that more sunlight exposure causes risk reduction.
What we can say is that the lifestyle constellation that appears protective — regular outdoor activity, physical exercise, consistent sleep patterns, adequate vitamin D — is supported by multiple converging lines of evidence, involves no meaningful harms when practised sensibly, and supports broader health across many conditions. The case for acting on it is strong.
For those who carry a genetic predisposition — a known mutation in LRRK2, GBA, or one of the other associated genes — the 2025 UK Biobank study offers particular encouragement: genetic risk is not destiny, and outdoor light exposure appears to offer protection even against a high polygenic background. Environment and behaviour are not helpless in the face of genes.
In Summary
Parkinson’s disease is predominantly shaped by environmental and lifestyle factors, not genetics alone. Light — the most ancient environmental signal we have — sits at the intersection of several of the key biological pathways involved: vitamin D synthesis, dopaminergic function, circadian regulation, and sleep.
The evidence is now sufficient to recommend, with confidence, that spending more time in natural daylight, supplemented by indoor lighting designed to support the body clock is a meaningful and accessible strategy — both for those who wish to reduce their risk of developing Parkinson’s, and for those already living with the condition.
Kicking the tyres at St Monica’s Trust next week
Tech looks great on paper, but you only really know if those fancy buttons and boxes really work when you see the person who will use it getting behind the wheel and taking it for a spin themselves.
So I’m looking forward to trying out some of the kit on show, and hearing what guests think about lighting at the St Monica Innovation day at the Chocolate Factory next week. The event, hosted by Sean Davey and Elise Towsend, is part of the Trust’s ongoing drive to transform the future of care through user-led innovation.
Spaces are strictly limited – please send me a message if you’d like to join.