Outdoor Artificial Light May Extend Light Sleep While Altering Deep Sleep Patterns

Higher levels of outdoor nighttime light (NTL) exposure were associated with modest changes in sleep architecture, particularly longer light sleep duration, whereas effects on total and deep sleep appear more complex and nonlinear, according to new research.¹ Exposure to artificial light at night has been widely implicated in circadian disruption and poor sleep outcomes, but evidence based on objective sleep measures remains limited.

The study, published in BMC Public Health, analyzed sleep data collected between 2017 and 2019 via Zepp Health smart bracelets from 4690 adults in China. Researchers linked individual sleep metrics to satellite-derived outdoor NTL exposure using NASA’s Black Marble product. NTL levels were categorized into tertiles and analyzed using mixed-effects regression models that adjusted for demographic, behavioral, and environmental factors, including age, sex, BMI, smoking and drinking status, income level, and ambient temperature.

Across the analytic sample, NTL exposure ranged from 1.00 to 32.45 W∙cm²∙SR⁻¹, with a median of 7.57 W∙cm²∙SR⁻¹ (IQR, 4.74–10.52). Participants in the highest NTL tertile were more likely to be female, younger (mean age, 44 years), and residents of higher-income, more urbanized cities. Although total and light sleep duration did not differ significantly across tertiles, deep sleep duration was longer among participants exposed to higher NTL levels in unadjusted analyses.

In fully adjusted models, a 1-standard deviation increase in NTL exposure (5.20 W∙cm²∙SR⁻¹) was associated with a 1.22-minute increase in light sleep duration (95% CI, 0.04–2.41). No statistically significant association was observed between NTL exposure and total sleep duration, with estimated differences of 1.17 minutes (95% CI, −0.11 to 2.45). Although deep sleep duration initially showed a positive association with NTL exposure, this relationship was lessened after full adjustment (0.10 minutes; 95% CI, −0.62 to 0.82).

The researchers highlighted a related viewpoint article; despite strong and consistent evidence demonstrating sleep’s fundamental role in physical, mental, and cardiometabolic health, sleep health remains widely under-recognized in global health agendas.² Coordinated efforts to raise public and policymaker awareness and to expand participation in the collection of high-quality sleep data were emphasized, with authors stating that robust, population-level sleep data are essential to inform evidence-based sleep health policies and improve overall population health.

In the new findings, subgroup analyses identified several populations with stronger associations between NTL exposure and prolonged light sleep.² Adults aged 65 years or older experienced an estimated 4.41-minute increase in light sleep duration per standard deviation increase in NTL exposure (95% CI, 0.91–7.90; P_interaction = .011). Larger effects were also observed among participants with low or normal BMI, smokers, nondrinkers, and those living in non–middle-income areas. In contrast, participants from middle-income regions showed smaller and nonsignificant associations.

Restricted cubic spline analyses revealed nonlinear exposure-response patterns. Light sleep duration remained stable at lower to moderate NTL levels before increasing at higher exposures. Deep sleep duration, however, increased modestly at moderate NTL levels, peaking at approximately 9-13 units before declining at higher exposure levels, indicating potential harm at excessive light intensities. The findings also showed that most participants were exposed to low-to-moderate NTL, with relatively few observations at the highest levels.

Ambient temperature also emerged as an important factor in the study. Researchers found that each 1-K increase in monthly average temperature was associated with a 0.71-minute reduction in light sleep duration (95% CI, 0.63–0.79). Sensitivity analyses using alternative exposure windows showed consistent results, with stronger associations observed when monthly average NTL was used rather than shorter exposure periods.

The researchers noted that the modest effect sizes may reflect limitations of satellite-based NTL as a proxy for personal light exposure, which does not account for indoor lighting, wavelength, or individual mitigation strategies such as blackout curtains. Nonetheless, the findings suggest that moderate outdoor nighttime light may subtly alter sleep structure, whereas higher exposure could compromise restorative deep sleep.

References

1. Yang H, Liu J, Chen Y, et al. Outdoor night light exposure and sleep structure, a panel study using consumer-grade wearables. BMC Public Health. Published online January 7, 2026. doi:10.1186/s12889-025-26132-3
2. Lim DC, Najafi A, Afifi L, et al. The need to promote sleep health in public health agendas across the globe. Lancet Public Health. 2023;8(10):e820–e826. doi:10.1016/S2468-2667(23)00182-2