Greater Daytime Light Exposure Linked to Better Sleep for Parkinson’s Disease

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A new study found patients with Parkinson’s disease have a lower daytime light exposure with a mean light intensity of 201.1 lux than controls with a light intensity of 337.7 lux.

Greater daytime light exposure and lower nighttime exposure are significantly linked to better objective sleep measures in patients with Parkinson’s disease, a new study found.1

“Our findings suggested that higher daytime light exposure was significantly associated with better actigraphic sleep measures, although previous randomized controlled trials suggested no significant changes in objective sleep measures following bright light exposure,” wrote investigators, led by Kenji Obayashi, MD, PhD, from the department of epidemiology at Nara Medical University School of Medicine in Japan.

Being exposed to too much light at night can cause circadian misalignment. Prior research found this could alter the core body temperature, melatonin secretion, and brain activity—as well as affecting sleep quality.2

Since patients with Parkinson’s disease often have circadian misalignment and experience frequent sleep issues, investigators sought to assess the association of daily light exposure with objective sleep measures in patients with and without Parkinson’s disease.1

Obayashi and colleagues conducted a cross-sectional study of 189 outpatients with Parkinson’s disease from the “Parkinson’s disease and the relationships with circadian biological rhythms and Sleep” (PHASE) study at Nara Medical University Hospital (October 2014 – April 2017) and 1101 community-dwelling older adults (≥ 60 years) as controls from the “Housing environments and health investigation among Japanese older people in Nara, Kansai region: a prospective community-based cohort” (HEIJO-KYO) study (September 2010 – April 2014).

Investigators evaluated sleep efficiency, wake after sleep onset, sleep onset latency, total sleep time, and fragmentation index by wrist light meters worn during the day and light meters placed in the bedrooms during the night. Participants wore the wrist light meter which measured in 1-minute intervals over 7 days for patients with Parkinson’s disease and 2 days for controls.

Investigators excluded daytime values < 1 lux since they were viewed as artifact data; if daytime values were < 1 lux for more than half of the daytime period, the data was declared missing. Obayashi and colleagues measured light exposure duration >1000 lux in intensity during the day.

Moreover, nighttime light exposure from bedtime to rising time was measured in < 2-minute intervals with light meters placed facing the ceiling 60 cm above the bed or at the head of a participant’s bed. The light meters were used for 7 nights and 2 nights for controls.

To assess sleep quality, participants kept a sleep diary for 7 consecutive days and underwent neurologic examinations.

The team found patients with Parkinson’s Disease had significantly shorter median duration of exposure to ≥ 1000 lux light than controls (24.7 minutes vs 50.5 minutes; P < .001). Additionally, patients with Parkinson’s disease had a greater median nighttime light intensity than controls (12.6 minutes vs 5.5 minutes; P < .001).

The multivariable analysis showed the highest quartile of light exposure to ≥ 1000 lux light during the daytime was associated with an 8% greater sleep efficiency (95% CI, 2.1 – 13.4; P = .008) and a shorter short wake after sleep onset by 36.9 minutes (95% CI, 13.4 – 60.3; P = .002) than the lowest quartile. At night, the highest quartile of mean light intensity had significantly reduced sleep efficiency by 6.8% (95% CI, 1.3 – 12.3; P = .016) and had a longer short wake after sleep onset by 24.1 minutes (95% CI, 1.8 – 46.4; P = .034), longer sleep onset latency by 0.7 minutes (95% CI, 0.3 – 1.0; P < .001), and a greater fragmentation index by -.3 log units (95% CI, 0.0 – 0.5; P = .006).

The study suggests patients with Parkinson’s disease have lower daytime light exposure with a mean light intensity of 201.1 lux (interquartile range [IQR], 101.2 – 305.7) than controls with 337.7 lux (IQR, 165.6 – 719) (P < .001). Furthermore, patients with Parkinson’s disease had higher nighttime exposure with light intensities of 2.0 (IQR, 0.5 – 7.8) than controls with 0.7 lux (IQR, 0.1 – 3.3) (P < .001)

Moreover, objective sleep measures, except for sleep onset latency, were worse in patients with Parkinson’s disease than controls (mean sleep efficiency: P < 0.001; wake after sleep onset: P < .001; fragmentation index: P < .0001) and mean total sleep time: P < .001). For patients with Parkinson’s disease, greater daylight exposure and lower nighttime light exposure were significantly linked to better objective sleep measures, even after adjusting for confounders such as daytime physical activity and disease stage.

Limitations investigators highlighted included the cross-sectional design prohibiting them from making causal inferences, wrist tremors, dyskinesia, and REM sleep behavior disorders affecting actigraphy data, the control group only having light exposure and sleep measures assessed after 2 days, and light meters being invalidated. Other limitations pointed out included not randomly selecting the control group, only including a Japanese population in the sample, not adjusting for potential confounders such as disease severity and sleep disorders, cataracts, or prior cataract surgery, and the Parkinson’s disease diagnosis not being supported by myocardial scintigraphy.

“Importantly, independent associations of daytime and nighttime light exposure with objective sleep measures were detected,” investigators wrote. “Although the present study treated light intensity, it would be important not only intensity but timing and wavelength when considering effect of light on sleep.”


  1. Obayashi K, Saeki K, Tai Y, et al. Daily light exposure profiles and the association with objective sleep quality in patients with Parkinson's disease: The PHASE study. Sleep. Published online February 8, 2024. doi:10.1093/sleep/zsae036
  2. Cajochen C, Münch M, Kobialka S, et al. High sensitivity of human melatonin, alertness, thermoregulation, and heart rate to short wavelength light. J Clin Endocrinol Metab. 2005;90(3):1311-1316. doi:10.1210/jc.2004-0957