Spending more time outdoors has long been one of the most effective ways to help reduce a child's risk of developing myopia. But what happens after the sun goes down?
A new population-based study suggests that another aspect of the light environment—outdoor artificial light at night (ALAN)—may also play a role.
Researchers in Hong Kong found that children living in neighborhoods with higher levels of nighttime outdoor lighting (i.e street lights) were more likely to have myopia and more likely to develop it over the next three years.
First, let’s get some background on this topic.
Childhood myopia is becoming increasingly common around the world, with researchers estimating that nearly half of the global population could be affected by 2050 if current trends continue.
- Beyond the need for glasses or contact lenses, higher degrees of myopia can substantially increase the lifetime risk of sight-threatening ocular conditions such as retinal detachment, myopic macular degeneration, glaucoma, and cataracts.
Go on …
And while genetics certainly play a role, environmental factors have become a major focus of research over the past decade. In particular, spending more time outdoors has consistently been shown to reduce a child's risk of developing myopia.
That raises an interesting question: If daylight helps protect the eye, could excessive artificial light at night (ALAN) have the opposite effect?
So, what did the researchers actually do?
To find out, investigators analyzed data from the Hong Kong Children Eye Study, a large population-based cohort that followed children for 3 years.
Rather than asking families to estimate nighttime light exposure, the researchers used time-varying satellite imagery to measure outdoor artificial light levels within a 500-meter radius of each child's home.
Then: They looked at whether greater nighttime light exposure was associated with both existing myopia at baseline and the development of new myopia during follow-up.
- To note: These analyses adjusted for factors already known to influence myopia risk: age, sex, near-work activities, time spent outdoors, parental myopia, family income, and other socioeconomic variables.
So, who took part in the study?
The cross-sectional analysis included 19,114 children (average age: 7.4), with males making up 52.8% of the study population.
For the prospective portion of the study: Researchers followed 2,558 children who were not myopic at baseline over a 3-year period to determine whether higher outdoor ALAN exposure increased their likelihood of developing myopia.
Myopia was defined as a cycloplegic spherical equivalent of -0.50 diopters or worse, using cycloplegic refraction to minimize accommodative error.
… and what did they end up finding?
The biggest takeaway: Children exposed to brighter outdoor nighttime lighting were more likely to develop myopia.
After accounting for demographic, behavioral, parental, and socioeconomic factors, every interquartile increase in outdoor ALAN exposure was associated with:
- 8% higher odds of having myopia at baseline (OR 1.08; 95% confidence interval [CI], 1.02-1.15)
- 26% higher risk of developing myopia during the 3-year follow-up (HR 1.26; 95% CI, 1.16-1.37)
The researchers also uncovered something particularly interesting.
Tell me more.
Instead of finding a simple linear relationship, they observed a nonlinear exposure-response pattern.
Myopia risk remained relatively stable at lower levels of nighttime light exposure before increasing much more sharply once exposure exceeded approximately 93.08 nW/cm²/sr.
Age also appeared to matter.
Children 7.35 years and older showed a significantly stronger association between outdoor ALAN exposure and incident myopia than younger children, suggesting that susceptibility may differ across stages of visual development.
Anything else worth mentioning?
One of the most intriguing findings wasn't simply that higher nighttime light exposure was associated with myopia—it was that the relationship appeared to have a threshold effect.
Rather than increasing steadily with every rise in outdoor ALAN, risk accelerated once exposure surpassed a certain level.
- If future studies confirm that threshold, it could eventually help inform urban lighting policies and environmental health recommendations aimed at protecting children's vision.
How interesting …
Indeed. Also an interesting finding to note: While this isn't the first time researchers have explored whether nighttime lighting influences refractive development, it is one of the first large longitudinal studies to examine outdoor artificial light exposure.
- Earlier studies investigating night lights in children's bedrooms generated considerable interest after reporting a possible association with myopia
- However, subsequent studies were unable to consistently replicate those findings, leaving the question unresolved
And more recently: Researchers have shifted their attention toward broader environmental light pollution.
So with that in mind …
The current study builds on previous work by examining objectively measured outdoor nighttime lighting over several years rather than relying on parental questionnaires or single-time-point assessments.
- Exactly how artificial light at night might influence eye growth remains uncertain.
Did investigators propose any potential explanations?
Indeed. The authors pointed to growing evidence that nighttime light exposure can disrupt circadian rhythms and suppress melatonin secretion.
And experimental studies have also suggested that normal circadian signaling may help regulate axial eye growth, raising the possibility that chronic disruption could contribute to abnormal refractive development.
Still, these mechanisms remain hypothetical and require further investigation.
Noted. Now circling back to this study—any limitations worth noting?
Like all observational studies, this could not establish that outdoor artificial light at night causes myopia.
- Although the researchers adjusted for a wide range of known risk factors, unmeasured confounding is still possible.
The study also estimated outdoor light exposure using satellite imagery rather than measuring the amount of light each child actually experienced indoors.
- Factors such as curtains, blinds, bedroom orientation, and nighttime screen use weren't captured.
Finally: Because the study population consisted primarily of Hong Kong Chinese children living in a densely populated urban environment, the findings may not apply equally to children living in rural settings or other parts of the world.
And what do the experts make of these findings?
Per the researchers: Their findings add to growing evidence that the overall light environment—not just daytime outdoor exposure—may influence childhood eye development.
At the same time, however, they stressed that the results shouldn't change current clinical practice.
For now, interventions with the strongest evidence remain those already recommended for children at risk of myopia, particularly increasing daily outdoor time and using established myopia-control strategies when appropriate.
They also noted that additional longitudinal studies, replication in other populations, and mechanistic research will be needed before outdoor ALAN can be considered a modifiable risk factor.
Take home.
A new longitudinal study found that children living in neighborhoods with higher levels of outdoor artificial light at night were more likely to have myopia and more likely to develop it over 3 years.
While the findings don't prove that nighttime light exposure causes myopia, they highlight another environmental factor that may contribute to childhood refractive development.
Additional studies will be needed to determine whether reducing nighttime light exposure could one day become part of broader myopia prevention strategies.