Unraveling the Impact of Environmental Factors on Sleep Quality and Parkinson Disease

Environmental changes, including those attributable to climate change, have increasingly been examined for their effects on sleep,¹ with implications for various health conditions including neurodegenerative disorders.² Nonmotor symptoms associated with Parkinson disease (PD), particularly sleep disturbances, greatly affect quality of life.³ In the following, we review the effects of various environmental factors—extreme temperatures, air and noise pollution, chemical exposures, and heavy metals—on both sleep and PD.

We examined studies published over the past 5 years (2019 to 2024) to provide a comprehensive overview of recent findings. By synthesizing this literature, we aimed to elucidate how these environmental stressors interact with sleep disturbances and contribute to the increased risk and morbidity of PD, thereby identifying potential areas for future research and intervention.

We also emphasize the notable research gap concerning the interaction between environmental stressors and sleep disturbances in people with PD. Given that sleep constitutes approximately one-third of an individual’s life and that sleep disturbances can impair quality of life considerably in people with PD, addressing this research gap is crucial. Furthermore, sleep disturbances often serve as early indicators or prodromal symptoms of PD, as evidenced by conditions such as rapid eye movement (REM) sleep behavior disorder. Bridging this research gap could lead to advancements in both preventive and therapeutic strategies for managing PD.

Sleep and Environmental Factors

Adequate sleep quality and volume are essential for maintenance of human health. Newborns spend almost 75% of the day sleeping.⁴ Although this percentage decreases as humans age, adults still spend about 33% of the day sleeping.⁴ Disturbances in sleep can impair childhood development, lead to decreased quality of life, and result in major health consequences including an increase in all-cause mortality.^5,6

In general, chemical exposure has been found to negatively affect sleep. Chemical exposure encompasses heavy metals (eg, lead, mercury, and cadmium) and pesticide exposure. Lead exposure can come from various sources, such as water pipes, contaminated food, paint, or industrial waste.⁷ Lead exposure affects sleep through modulation of various neurotransmitters.⁷ Higher cumulative lead level during childhood is associated with less sleep and greater sleep fragmentation in adolescents and poor sleep quality in adults.^8,9 The effects of mercury on sleep results from its accumulation in the pineal gland, which plays a central role in sleep regulation.¹⁰ In a Mexican cohort, mercury exposure did not strongly correlate with sleep disturbances in children but was associated with later sleep timing in adolescence.¹⁰ In adults, exposure to mercury has been associated with insomnia, excessive sleepiness, sleep disturbances, and daytime fatigue.¹⁰ Study results demonstrated a significant association between cadmium exposure and sleep disturbances in adults.¹¹ Pesticide exposure has been linked to short sleep duration, difficulty falling sleep, and worse sleep quality.¹²

Air, noise, and light pollution have all been associated with sleep disorders. Studies have shown that air pollution negatively affects sleep.¹³ Exposure to nitrogen dioxide and fine particulate matter in adults and exposure to ozone (O₃) and sulfur dioxide in children is associated with increased risk of sleep-disordered breathing, which likely results from airway inflammation and edema.^14,15 Although data on the effects of air pollution on sleep duration are inconsistent, higher pollution exposure generally correlates with increased sleep variability.^16,17 Air pollution is also associated with poorer sleep quality, insomnia, prolonged sleep latency, and excessive daytime sleepiness.^13,18-22

Research on noise pollution and its effects on sleep has produced mixed results. Studies examining aircraft noise near airports found associations with increased awakening probability, reduced sleep quality and efficiency, difficulty falling asleep, and frequent nighttime awakenings.^23-25 Road traffic noise, which is less intermittent than aircraft noise, was less strongly linked to sleep disturbances.^26,27 For instance, a study in Spain reported no significant effects of environmental noise on sleep,²⁸ and another study found that high environmental noise levels were associated with later bedtimes in adolescents but did not affect total sleep duration.²⁹ Furthermore, study results indicated that indoor noise is associated with lower sleep quality by affecting sleep efficiency, sleep onset latency, wake periods after sleep onset, and sleep fragmentation.³⁰

Light pollution is linked to increased sleep disturbances and poor sleep quality in both children and adults.^31-37 Reported disturbances include reduced sleep quality, shorter sleep duration, later bedtimes, decreased sleep efficiency, increased wakefulness after sleep onset, and heightened movement index.^{31,32,34,36,37} One study showed that indoor light pollution was associated with elevated white blood cell counts, suggesting increased systemic inflammation.³³ In another study, nighttime exposure to short wavelength light was shown to decrease levels of melatonin.³⁷ Melatonin is known to have anti-inflammatory properties, prompting the hypothetical correlation among light exposure, melatonin levels, and subsequent increased inflammatory state as evidenced by increased white blood cell counts.³³

Global warming has increasingly garnered attention, yet its effects on sleep remain underexplored.¹ Temperature plays a crucial role in sleep regulation, as thermoregulation affects sleep onset and melatonin secretion.^38,39 Elevated temperatures were found to impair slow-wave sleep, REM sleep, and overall sleep efficiency.^38,40,41 Lower intradaily temperature variability is associated with better sleep quality, including increased sleep efficiency, longer sleep duration, and reduced wake time after sleep onset.⁴² The effects of temperature on sleep are well-documented, but studies on how global warming influences sleep are limited. One recent article suggested that short-term heat exposure adversely affects sleep, but individuals in hotter regions may adapt over time.¹ Another study showed that indoor temperature may have a greater role in sleep compared with outdoor temperature.⁴³ As global temperatures rise, further research is essential to understand how these changes will affect sleep patterns.

PD and Environmental Factors

PD is a progressive neurodegenerative disorder characterized by abnormal α-synuclein deposits in the brain, which has been increasingly linked to various environmental factors. The effects of exposure to various chemicals and metals, air and noise pollution, extreme temperatures, and smoking on PD have been studied. Data showed a positive correlation between increased risk of PD and exposure to chlorinated hydrocarbon solvents, pesticides, and heavy metals (eg, manganese, lead).^44-53 Moreover, although rural living, proximity to bodies of water, and well water consumption were linked to an increased risk of PD, this association was likely driven by higher likelihood of pesticide exposure in these instances.^47,48,50

Data on air and noise pollution are less conclusive, with conflicting positive and negative associations reported between air pollution and PD^54-62 and a more convincing absence of a link between noise pollution and PD.^53,58 Ambient air pollution⁵⁸ and long-term exposure to solar radiation,⁵⁴ O₃,^54,60,61 or carbon monoxide (CO)^55,61,62 were associated with an increased risk of PD incidence and, in 1 study, increased PD-related overall morbidity and mortality.⁶⁰ Conversely, a recent meta-analysis pooling results from 13 reports found no statistically significant association between long-term exposure to small particulate matter (PM)_2.5 and PD, with a pooled relative risk of 1.06 (95% CI, 0.99 to 1.14).⁶² This finding was further supported by recent case-control studies by Toro et al⁵⁹ and Jo et al,⁵⁶ which showed no positive correlation between PM_2.5 or PM₁₀ and PD. The study by Toro et al⁵⁹ did not account for greenness (ie, the density of plant and tree coverage in a given region), and the study was conducted in one of the greenest cities in the Netherlands. Greenness has been shown to be a protective factor against PD and to mitigate the effects of air pollution,⁵⁸ potentially confounding the results of the Toro et al study. The study by Jo et al⁵⁶ found no significant correlation between PD incidence and exposure to PM_2.5, PM₁₀, O₃, sulfur dioxide (SO₂), or CO, but the O₃ levels reported in the study were significantly lower than those in the Israeli study. In other case-control studies, PM₁₀ exposure was associated with an increased risk of PD,^54,57 with a stronger effect observed in cases with a “definite” diagnosis of PD. Although the risk between air pollution and PD incidence is still unclear, these studies highlight the need for further research assessing this risk because air pollution is a modifiable risk factor.

Two case-control studies from Finland⁵³ and Israel⁵⁴ showed no correlation between extreme temperatures and PD incidence. In the Israeli study,⁵⁴ although ambient temperature did not appear to significantly affect PD risk and incidence in the overall population, newly immigrated populations (mostly from colder climates) showed a stronger correlation between PD incidence and exposure to O₃ (odds ratio [OR], 1.43 [95% CI, 1.05 to 1.95] vs OR, 0.95 [95% CI, 0.63 to 1.42] in nonimmigrants) and solar radiation (OR, 1.73 [95% CI, 1.25 to 2.38] vs OR, 0.67 [95% CI, 0.4 to 1.13] in nonimmigrants). Both ambient⁵⁸ and occupational⁵³ noise pollution and road proximity⁵⁸ were studied in 2 recent case-control studies and were found not to be correlated with PD risk. A retrospective cohort study by Yuchi et al⁵⁸ found a slight correlation between PD and noise levels in a unique subset of the population (ie, women) with a hazard ratio of 1.06 (95% CI, 1 to 1.13). However, given the previous negative studies and the minimally statistically significant hazard ratio, the relevance of this correlation is unlikely to be significant.

The effect of environmental factors should be considered alongside the impact of genetic and epigenetic factors (eg, SNCA or α-synuclein gene polymorphism^48,50). PD is believed to develop from the epigenetic modulation of gene expression through environmental factors, accounting for up to 85% of all PD cases.⁵⁴ A recent study showed that the best predictive model for PD development included a polygenic risk score alongside 7 other independent factors.⁶³ Future studies should also account for different PD subtypes, as they do not respond similarly to the same known protective factors, such as smoking^45,46,49 and coffee consumption.^45,49,64 Finally, most of the factors mentioned previously were studied in relation to PD but not REM sleep behavior disorder, which can be considered a PD prodromal phase, making this an interesting topic of research for future studies.

Discussion and Suggestions

Air pollution,^{13,18-21,54,58,60,61} heavy metals, (eg, lead)^7-10,44,52 and fluctuations in temperature^43,54 are each independently linked to poorer sleep quality and a higher risk of PD. People with PD often have reduced overall sleep efficiency and increased nighttime awakenings,⁶⁵ and sleep quality remains the main determinant of the overall quality of life in this population.³ Moreover, people with PD have a higher incidence of certain sleep disorders (eg, insomnia, REM sleep behavior disorder, restless legs syndrome) and any overlying sleep disorders can affect them more negatively given their baseline fragile sleep.⁶⁶ Improving sleep quality and reducing environmental factors that interfere with sleep quality are crucial for managing symptoms and enhancing well-being in individuals with PD. Modifiable factors, such as air pollution, can be mitigated by increasing green spaces and adopting measures to address global warming, which can help decrease pollutant exposure.⁵⁸ Furthermore, maintaining consistent, moderate temperatures (recommended 62° to 70° F) is beneficial for optimal sleep quality, and thus can potentially improve quality of life for people with PD. The Figure provides an overview of various sleep disorders associated with PD and their causes and neurologic changes.

Evidence suggests that alterations in the drainage function of the brain’s lymphatic and glymphatic systems during sleep may contribute to the failure of toxic protein clearance in neurodegenerative diseases such as PD.^67-69 Moreover, sleep deprivation has been linked to increases in Β-amyloid accumulation, interstitial fluid tau, cerebrospinal fluid tau, and α-synuclein, indicating that changes in circadian rhythms could be risk factors in aging-related neurodegenerative diseases.^70-73 Understanding the effects of these environmental factors on sleep in people with PD is vital, and addressing these environmental factors and their effects on sleep in people with PD is a much-needed area of research. Tackling climate change and global warming, which drive many adverse environmental changes, could have substantial benefits for sleep quality, PD management, and overall public health.