Time outdoors and the prevention of myopia: School-based implementation works

Abstract

Increased time outdoors has been convincingly associated with less myopia in observational studies. School-based interventions using increased time outdoors have been shown to reduce myopia onset, even in large-scale implementation. While each of the studies on which these conclusions are based has limitations, the consistent evidence has established that increasing time outdoors provides a powerful approach to controlling the current epidemic of myopia. A causal pathway for protection has been confirmed in studies on experimental myopia in animals, involving increased release of retinal dopamine by brighter outdoor light during daylight hours. This does not preclude involvement of other mechanisms, and initial results suggest that interventions to increase lighting in classrooms, or to provide environments rich in higher spatial frequencies similar to natural environments outdoors, may provide synergistic benefits. Whether increased time outdoors slows myopia progression is more controversial. Observational epidemiology and intervention trials have given inconsistent evidence, but studies on seasonal variations suggest that progression may also be regulated by light exposures and near workloads. At present, the evidence is sufficient to consider increased time outdoors as an adjunct therapy for myopia control, but primary control should be based on well-validated interventions. As increased time outdoors is increasingly used, one research priority should be to establish optimal protocols for school-based interventions. Another should be to explore the impact of these interventions on learning outcomes, since modern educational theory suggests that more play-based learning in the early years may lead to better long-term educational outcomes. The broader impacts on student mental and physical health should also be investigated. Little attention has so far been paid to how to reduce myopiagenic pressures associated with schooling, without compromising educational outcomes. This is an important area for future research, which should involve collaboration between experts in vision and ophthalmology and experts in education.

Introduction

Myopia has been known for at least 2000 years, since Aristotle first gave the condition its name. In 1652, Kepler described the optics of the myopic eye and speculated that his own myopia had been induced by education. However, it was not until the mid-19^th century that the retinoscope and ophthalmoscope were invented, enabling serious epidemiological studies.[1]

By 1882, Cohn had documented a high prevalence of myopia in senior high schools in Germany, as several European countries expanded their education systems, supporting the idea that education caused myopia.[2] This was not a population-wide epidemic because of low participation in education, but the change was so dramatic that the Strasbourg Manifesto recommended reforms of education, including increased sports activity and better school lighting, that appear to have had positive effects.[3]

Eighty years later, Sorsby et al. demonstrated that there was a high concordance of myopia in twins,[4] and, after a brief but vigorous polemic over whether there was a rapid increase in myopia prevalence in Inuits,[5] the idea that myopia was largely genetically determined rapidly became textbook orthodoxy. This perspective remained dominant until the rapid rise of myopia in parts of East and Southeast Asia led to the argument that myopia could not be genetically determined, because the epidemic had developed too rapidly to be explained by genetic change.[6] In <10 years, this had become the dominant theory,[7,8] accepted by many, if not all, myopia geneticists.[9] The importance of environmental factors was recently emphasized by the marked, but largely transient, increases in myopia associated with the COVID-19 pandemic.[10]

The Rise of Environmental Factors

The shift away from genetic determination led to an explosion of interest in modifiable risk factors, with an emphasis on two key causal associations[11] – a link to education, which promotes the development of myopia[12] and a link to time spent outdoors, which protects from myopia.[13] This shift also led to greater interest in the extensive evidence on experimental myopia in animals induced by environmental manipulations.[14,15]

Particular attention has been devoted to time outdoors because of its potential for prevention. The first significant evidence of a link had come from studies on the progression of myopia in Finnish children.[16] Mutti et al.[17] then reported that more time on sports was associated with a lower prevalence of myopia. They speculated that this association might represent a protective effect, although they considered a clinical trial involving randomized levels of sports participation to be impractical.

Four years later, two abstracts[18,19] were presented at the ARVO meeting in 2006, which further documented links between time outdoors and myopia. In the first follow-up paper to these Abstracts, Jones et al.[20] reported longitudinal data on children who were not myopic at their Grade 3 examination, with follow-up data available in Grade 8. Incidence of myopia over this period was greater among children with myopic parents and among those whose parents reported that their children had limited participation in sports and outdoor activities. Children with more than 14 h/week outdoors had a lower prevalence of myopia, even if they had myopic parents, but variations in near-work time had little impact.

In the second follow-up paper, Rose et al.[21] reported cross-sectional associations between more near-work and a higher prevalence of myopia, and more time on outdoor activities and less myopia. Multiple regression analysis showed that there was a competitive interaction between near-work time and time outdoors, with children who performed high levels of both near-work and time outdoors showing a low prevalence of myopia. In contrast, those who reported a combination of high levels of near work and little time outdoors had a much higher prevalence. This study did not find significant associations between time on indoor sport and myopia. It also proposed that the effect might be due to brighter light outdoors stimulating dopamine release.

A companion paper[22] reported on 6–7 year-old children of Chinese ancestry growing up and being educated in Singapore or Sydney. The children in Sydney were much less myopic than those in Singapore, with much more time spent outdoors in Sydney. In 2013, follow-up data reported[23,24] associations of 5-year incident myopia with increased near-work and less time outdoors. Many other papers have since reported similar associations, including data from Singapore[25] and Taiwan.[26,27]

These previous studies generally used questionnaire estimates of near work and time outdoors, but a distinct set of evidence led to similar conclusions. Sherwin et al. used conjunctival ultraviolet-activated fluorescence (CUVAF) to give a more objective, albeit only semi-quantitative measure of cumulative time spent outdoors. A general correlation between more CUVAF (thus more time outdoors) and less myopia was established.[28]

From Association to a Causal Connection

Observational epidemiological evidence does not establish a causal connection. To prove causality, the gold standard approach is to carry out a randomized clinical trial, where unknown confounding factors will be randomized between the control and the intervention groups. However, randomization is difficult in many cases,[29] which often leads to reliance on less definitive evidence.

Five significant intervention trials have assessed the impact of increased time outdoors [Table 1].[30,31,32,33,34] Results for the Sujiatun Study are reported only for the subsample that underwent cycloplegic refraction.[32] The STORM study had two experimental arms, which are reported separately.[31] These arms were allocated different regimes with different targets for time outdoors, but objective measures suggested very similar exposures in the two arms. Despite their significant differences in design, all studies reported significant reductions in the onset of myopia, significant reductions in myopic shift in refraction, and, with one exception, significant reductions in axial elongation.

Table 1.

School-based intervention trials

	Number of schools	Randomisation	Objective measure of time outdoors	Reduction in myopia onset (%)	Reduction in myopic shift in refraction (%)	Reduction in axial elongation (%)
Wu et al. (2013)[34]	2	No	No	52	34	NA
He et al. (2015)[30]	12	Yes	No	23	11	3 (NS)
Jin et al. (2015)[32]	4	No	No	56	63	24
Wu et al. (2018)[33]	16	Yes	Yes	17	26	15
He et al. (2022)[31]	24	Yes	Yes	(1) 16	(1) 23	(1) 15
				(2) 11	(2) 16	(2) 12

NA=Not available, NS=Not significant. In the paper by He et al, there were two experimental arms, which they called test I and test II

There was a clear relationship between duration of exposure and prevention of incident myopia [Figure 1]. Two studies, the GOALS study from Guangzhou[30] and the Sujiatun study from northern China[32] had similar targets for time outdoors (45 min vs. 40 min) but gave quite different outcomes. Animal studies suggest that light exposures are more effective as repeated small exposures rather than a block.[35,36] The less effective intervention in the GOALS study was delivered as one block, while the more effective intervention in the Sujiatun study was delivered as two blocks of 20 min. This provides a possible explanation of the different outcomes.

Figure 1.

Reduction in myopia onset with increasing time outdoors. Data taken from Wu et al. 2013,[34] He et al. 2015,[30] Jin et al. 2016,[32] Wu et al. 2018[33] and He et al. 2022[31]

A major limitation of previous epidemiological studies, apart from the work on CUVAF, is that time outdoors was based on subjective estimates. Recently, more objective methods have been developed, with different strengths and weaknesses.[37,38] In principle, devices such as the Clouclip,[39] that are fitted to a child’s spectacle frame, measuring light intensity along the line of sight, are most likely to provide direct measures of exposure, but a major disadvantage is that they require non-myopic children to wear something, such as spectacle frames, to which the device can be attached. Two intervention studies carried out objective measurements of exposures, and both reported difficulties in achieving the exposure specified in the trial.[31,33] This has important implications for translation into practice, because it may be necessary to specify interventions in terms of concrete changes to the school program, such as allocated time-slots for time outdoors, rather than in terms of aspirational targets.

Translation into Practice

The translation of these results into practice was rapid. Singapore adopted increased time outdoors as part of its National Myopia Prevention Plan in 2008 but placed emphasis on the role of parents rather than schools. Some success has been reported, but major changes have not been documented.[40]

In 2010, Taiwan began implementing the Tian Tian 120 program (TT120 Program), in which schools aim for 120 min outdoors each day. This program began as a recommendation to schools, and even today, participation is not mandatory. The long-term trend to increasing levels of reduced visual activity (a proxy for myopia) in primary schoolchildren was reversed, although the overall level remained high.[41]

The TT120 program was not applied to preschools, even though the prevalence of myopia in preschools in Taiwan was known to be high.[42,43] This gap was filled by the Yilan Myopia Prevention and Vision Improvement Program, which has been running a modified version of the TT120 program in preschools. The impressive results show that the prevalence of myopia has been almost halved.[42,44,45,46,47] Over the 2 years introductory period, there was a step-wise reduction in prevalence, and the students are now being followed into the primary school years. Whether the gains are maintained remains to be established.

Increased Time Outdoors Can Be Used to Slow the Onset of Myopia in Preschool and School Settings

Each of the studies discussed above has limitations, but the consistent associations between more time outdoors and less myopia in epidemiological studies, combined with the reductions in incident myopia in intervention studies, and success in wider implementation studies, collectively provide strong evidence that increasing time outdoors in schools can be used to control the current epidemic of myopia. Recent projections based on the effects seen in the Yilan studies suggest that major reductions in prevalence may be achievable in practice.[48]

Because the delayed onset of myopia will lead to less time for subsequent onset of myopia and for myopia progression, and because the rate of myopia progression declines with age,[49,50] earlier onset is likely to lead to a reduction in the final prevalence of myopia and also high myopia. Where the prevalence of myopia has increased, the prevalence of high myopia has disproportionately increased,[51,52] and this excess high myopia is in principle preventable with increased time outdoors. At this stage, the evidence suggests that the pathological consequences associated with high myopia are largely dependent on structural changes in the choroid and sclera of highly myopic eyes, and thus it seems likely that reductions in high myopia will lead to reductions in pathological outcomes. Longer-term research is required to confirm these conclusions, but there is no reason to delay action until this research is completed.

Increasing time outdoors in schools has already reached the stage of large-scale implementation. Once this stage has been reached, control groups are rarely used since it is both unethical and generally impractical to restrict the scope of the intervention, and comparison is generally based on historical data, which will need to be regional to ensure validity. Controlled variations in implementation to establish the most effective approach may be important. While the validity of the approach has been established, there are a number of issues that remain controversial.

The Mechanism of Protection

In establishing causality, it is important to identify a causal mechanism.[29] For the protective effect of time outdoors, Rose et al.[21] and French et al.[13] proposed one based on the increased release of dopamine from the retina by brighter light outdoors. Dopamine plays a fundamental role in light adaptation,[53] and release is high during the light phase.[54,55] Light-stimulated retinal dopamine release has been demonstrated,[56] as has flicker-stimulated release.[57] The ability of dopamine and dopamine agonists to slow the rate of axial elongation has also been demonstrated in animals.[58,59] This will be referred to as the light-dopamine hypothesis.

The ability of bright light to slow the development of form-deprivation myopia has been demonstrated in chickens,[60,61,62] tree shrews,[63,64] mice,[65] guinea pigs,[66] and rhesus monkeys.[67] Compensation for negative lenses was also slowed in chickens[68] and guinea pigs,[69] although the compensation point was, as expected, not affected. It has also been shown that the protective effect of light can be blocked by the administration of dopamine antagonists.[62,68] The only exception to the observed protective effects of bright light is one report that it did not influence the development of lens-induced myopia in monkeys.[70] This negative result is an outlier, unexpected given that dopamine’s functions are likely to be conserved across species. This observation, therefore, requires further confirmation before concluding that the regulation of eye growth in primates is different, particularly since there is suggestive evidence of efficacy from a small study on primates.[71]

The light–dopamine hypothesis proposed a light-induced brake on axial elongation. It did not propose a mechanism for guiding normal refractive development toward a specific endpoint. The evidence supports such a mechanism, since, in the lens-induced myopia paradigm, under bright light, it took longer to achieve the compensation point, consistent with inhibition of axial elongation.[68] In experiments using the lens-induced hyperopia paradigm,[68] compensation was achieved more rapidly under bright light, again consistent with slowed axial elongation.

Other Mechanisms for Protection

This evidence does not preclude the possibility that other differences between indoor and outdoor environments may also play a role, and many potential differences have been suggested. It is important to note that exposures can be much more rigorously controlled in animal experiments than is generally possible with children, and extrapolation to human situations requires careful consideration. When positive results are obtained, it is important to establish if similar exposures might be experienced by children. For example, prolonged exposure to monochromatic red light has been shown to have strong effects on the development of refraction.[72] Such exposures are unlikely to have much relevance to the aetiology of myopia, both because natural exposure to monochromatic light is rare, and because any exposure is unlikely to be prolonged. However, a strong effect of monochromatic light may be of potential use in prevention.[73]

One of the first alternatives considered was a role for Vitamin D, given that UV exposures are inherent in spending time outdoors. After an initial report,[74] numerous studies have consistently found an association between myopia and lower vitamin D levels. However, a causal mechanism has not been established. Three papers have cast doubt on a causal role for Vitamin D, through a detailed time-course analysis[75] and a Mendelian randomization approach.[76,77] The data, therefore, do not suggest that Vitamin D plays a significant role.

Whether physical activity protects from the development of myopia is more controversial. A lot of physical activity is performed outdoors, and thus it overlaps with time outdoors. Rose et al.[21] reported that there was no association between indoor sport and myopia, but several studies have since reported an association between more sport and less myopia, without controlling for time outdoors.[78] Recently, a detailed analysis of the STORM results has suggested that there may be an independent protective effect of physical activity,[79] so this question has still to be resolved. An independent effect of physical activity would be compatible with the light-dopamine hypothesis, because physical movement translates spatial into temporal contrast, which stimulates dopamine release.

A potentially complementary approach would be to increase the light intensity within classrooms. In a trial in rural northern China, classrooms were rewired to improve lighting conditions from under 100 Lux to over 400. After 1 year, incident myopia was reduced from 10% in the control group to 4% in the intervention group, with statistically significant reductions in myopic shifts in refraction and axial elongation.[80] A related approach was to substitute a more natural spectrum of light for normal classroom lighting.[81] This produced a 20% reduction in incident myopia over 1 year, a 22% reduction in myopic shift in refraction, and 7% reduction in axial elongation. Another variation is the development of a bright classroom, designed as a stand-alone classroom, which achieves exposures of up to 7000 Lux through plastic walls and ceiling.[82] Unfortunately, it has not been tested for efficacy.

More recently, Flitcroft et al.[83] suggested that the higher spatial frequency content of natural scenes outdoors compared to man-made spaces indoors might be a significant protective factor, rather than higher light intensity. This hypothesis is consistent with the development of myopia in the form-deprivation model, and with the ability of Bangerter foils, as high spatial frequency filters, to induce experimental myopia.[84,85] A proof-of-concept trial involving outdoor scene classrooms has given positive results.[86]

So far, attempts to vary exposures within classrooms have provided suggestive evidence that varying exposures within the classroom may be effective in delaying the onset of myopia. Further research is clearly needed.

A Challenge to the Role of Time Outdoors and Light

In general, the role of time outdoors and light has been accepted, but one paper has argued that these exposures are not the key to the protective effect.[87] One of its main arguments is that the prevalence of myopia in 12-year-old children in Singapore is much higher than that in Norway, although the reported time spent outdoors is not very different. The flaw in this argument is that the development of myopia is cumulative, and in older children, the prevalence is a reflection of the sum of all previous exposures. Only the change that occurs in a given year can be directly related to the contemporaneous exposures. A previous study comparing 6–7 year-old children of Chinese ethnicity has shown that the prevalence of myopia in Singapore is much higher than in children growing up in Sydney, with a much lower time spent outdoors in Singapore.[22] The article from Norway provided estimates of time spent outdoors by younger children in Norway, which suggest that they are more like the children of European ancestry in Sydney, spending 2 or more hours outdoors/day. The authors recognized that this might support the idea that exposure to time outdoors and light prevents myopia, but nevertheless suggested that “the early onset of myopia as observed in many East Asian populations may be driven by genetic predisposition rather than environmental factors,” although genetic factors that explain the differences in prevalence between East Asian and European samples have not so far been detected.[88,89,90,91] Thus, the results from Norway in fact seem to be consistent with a protective effect of time outdoors due to light exposures. The authors also suggested that the effective control of myopia at these northern latitudes was inherently not compatible with a role for time outdoors and light, given the seasonal deprivation of light outdoors, but this could simply mean that evolution has designed a mechanism robust enough to maintain control in even quite extreme environments. However, it should be noted that in slightly more northern latitudes, a rapid increase in the prevalence of myopia was induced in some Inuit populations by the move of the populations into settlements, and the provision of quite limited education. In contrast, the pressures associated with education in Norway seem comparable to European norms, yet the situation has remained stable. This may mean that there is a very severe level of seasonal deprivation of light, seen only at the most extreme latitudes, at which the eye growth control mechanism fails.

What Outdoor Exposures are Required to Protect from the Development of Myopia

A common recommendation is to aim for 2 h/day, but it is not clear how this value was settled on. It probably derives from Figure 4 of Jones et al. 2007,[20] which shows that children with >14 h/week of sports (and time outdoors) had a substantially lower prevalence of myopia than those with less. A similar figure can be derived from the results in Rose et al.[21] At the highest tertile of time outdoors, from 2.7 + h/day in younger children to 2.8 + in older children, the impact of near work was effectively abolished. These results suggest that there may be some benefit in aiming for somewhat more time outdoors than the current recommendation of 2 h/day– an idea that is also supported by the results of intervention trials [Figure 1]. The line of best fit extrapolates to the complete elimination of myopia onset at close to 160 min/day. It is, therefore, interesting that recent media reports from China have noted that three primary schools had no myopia, in the most recent rounds of standardized school screening (www.chinaview.cn/20251103/a75f302f15d444cea6f9c31d0222b1b1/c.html). These schools were all boarding schools in Yunnan Province, and all had a policy of 3 h outdoors per day. Thorough examination of the school screening results may reveal other interesting patterns.

Establishing light intensity requirements is more complex because light intensity outdoors varies considerably throughout the day and regionally and seasonally. Measured intensity also depends on the orientation of the light meter relative to the sun, with light intensities higher when the light meter is pointed upward and lower when the light meter is pointed down. In fact, humans rarely spend much time looking at the sky, or at the horizon, but tend to look slightly downwards and at near objects and are therefore exposed to lower light intensities than are potentially available.

Light intensities under normal laboratory conditions are in the range of 200–500 Lux, whereas outdoors, light intensities of 10,000 up to 100,000 Lux or more are common during daylight hours. Increasing the light intensity under laboratory conditions to levels that would be naturally encountered outdoors slows the development of experimental myopia, as discussed previously. The animal experiments suggested that exposures of at least 10,000 Lux for several hours are required to produce inhibition, but these intensities were generally measured with the lux meter pointed upward, rather than along the line of sight.

The STORM study measured objective light exposures with wrist-watch light meters, where the orientation relative to the line of sight is variable. Detailed analysis suggests that at least 2000 Lux for at least 15 min is required for detectable effects.[92] This intensity is much lower than required in animal studies but is consistent with animal studies on the minimum duration of exposures. It is also consistent with results obtained using Actiwatch devices on 10–15 year-old children in Brisbane, Australia, which found that the exposures were generally low, with only 20–30 min/day at >5000 Lux.[93] The difference in light intensities required for effects on experimental myopia in animals and in humans can probably be explained by the different orientations of the light meters. The greater duration may be explained by the fact that the animal experiments involve continuous myopiagenic stimulation. In contrast, in children, the stimulus that promotes myopia is likely to be discontinuous, especially while outdoors.

These observations are broadly consistent with results obtained from animal models, which suggest that as little as some minutes of exposure to a stimulus is required to generate a response.[94] However, removal of diffusers and lenses for a period of minutes to hours can lead to restoration of normal eye growth.[35,36,95] One of the complications is that removal of the devices completely removes the myopiagenic drive. This is unlikely to happen when children go outdoors, since most children will still engage in some near vision activities. Animal experiments have suggested that breaking the exposure up into multiple shorter periods may be more effective that a single exposure of longer duration.[96] Other animal experiments have suggested that the time at which exposure occurs relative to circadian rhythms may also be important.[97] There are many variations that can be explored in school-based interventions, but it must always be remembered that ensuring compliance with complex interventions in a school setting is difficult.

Does Increased Time Outdoors Protect from Myopia Progression?

Before addressing this question, it is important to discuss the meaning of myopia progression. Traditionally, the term has been used to describe the fact that once myopia appears, it gets more severe until stabilizing in young adults. More recently, the term progression is being used to describe myopic shifts in refraction both before and after the onset of myopia, despite the fact that age-for-age, myopic shifts in refraction and axial elongation are much higher in myopes than in their hyperopic peers, particularly in younger children. In addition, recent work has shown that the acceleration of axial elongation and myopic shifts in refraction actually occurs before the onset of myopia,[98,99] and even before 0D, or emmetropia.[100,101] This suggests that there is a fundamental shift in retinal processing in the pre-myopic range. It is therefore vital to analyze changes in refraction and axial length in relation to baseline refractive status, or important differences may be obscured. For this reason, we believe that the term “progression” should only be used for myopic shifts in refraction in myopes. It is not hard to use more precise language, referring to changes, increases, and decreases in SER or axial length in other cases.

Some of the early evidence indicated increased time outdoors might protect from the progression of myopia.[16] However, detailed analysis of the CLEERE data indicated that increased time outdoors did not affect the progression of myopia.[102,103] There are some difficulties in drawing conclusions from observational studies, including that those with myopiagenic behaviors will generally be over-represented in myopic samples, which may limit the range of variation in time outdoors, making it harder to see positive effects. Results from intervention trials, in which behavioral change is imposed, are likely to produce more definitive results. This analysis has not been performed for the GOALS trial, due to the low number of myopes at baseline, but both of the trials from Taiwan reported a slowing of myopic shifts in refraction in myopes.[33,34]

There is independent evidence that the rate of myopic shifts in refraction in myopes can be environmentally regulated, since the myopic shifts in refraction are generally more rapid in winter than in summer, with similar changes in axial length. Eight studies have reported on this issue [Table 2].[104,105,106,107,108,109,110,111] Rhythmicity has been reported in non-myopes as well as myopes. One exception was a study from Japan on myopes that reported rhythmicity in axial length, but, if anything, myopic shifts in refraction were higher in summer than in winter. Another study from Singapore did not report seasonal rhythms but reported an effect before the examination period.[112] The negative finding could be explained by the limited seasonal variation in Singapore. More studies on rhythmicity in different locations correlated with seasonal differences in exposures might establish if these rhythms correlate with exposures.

Table 2.

Seasonal rhythms in myopic shifts in refraction

	Location	Parameter	Summer change as a % of winter	Sample
Hecova et al. (2023)[110]	Czech Republic	AL	<1	Population. Rhythm in both myopes and nonmyopes
Tsai et al. (2019)[111]	Taiwan	SER	Myopes 80
			Emmetropes 65
			Hyperopes 61
Gwiazda et al. (2014)[109]	USA	SER	40	Myopes
Donovan et al. (2012)[104]	China	SER	58	Myopes
		AL	81
Fulk et al. (2002)[107]	USA	SER	47	Myopes
Fulk et al. (1996)[106]	USA	SER	45	Myopes
Goss et al. (1998)[108]	USA	SER	54	Myopes
Fujiwara et al. (2012)[105]	Japan	SER	125	Myopes
		AL	81

These rhythms are what would be expected if time outdoors were the regulatory factor, but this is not proven. There is some evidence that patterns of behavior in children from East Asia differ from those in Western countries, with the latter spending more time outdoors on weekends and the school holidays than during the school week.[113,114] These patterns may be different in children of Chinese or East Asian ancestry, since there is a tendency for the suppression in the summer to be weaker in the East Asian studies. One study has indicated that Chinese children spend less time outdoors on weekends than during the school week.[115] This suggests that programs to encourage Chinese children to engage in outdoor activities outside of school hours, on weekends, and during the school holidays, may also be beneficial.

An important detailed study[93] followed 101 Australian children aged 10–15 (41 myopes and 60 nonmyopes) over 18 months, monitoring axial elongation, and light exposures with an Actiwatch. The conclusion was that greater light exposure was associated with less axial elongation in both myopes and nonmyopes.

These lines of evidence suggest that recommendations to increase time spent outdoors might be useful for children who are already myopic, and there is some evidence that control interventions were less effective during the COVID-19 epidemic, when time outdoors were restricted.[116,117] However, at this stage, recommendations of this kind should be regarded as supplementary to the use of well-validated clinical interventions.[118]

Are There More General Benefits from Increased Time Outdoors?

An important consideration in developing myopia prevention and control programs is whether there are additional benefits from encouraging children to spend more time outdoors. Only a few studies that have explored this possibility in relation to physical health and to learning outcomes.[119,120] There is, however, a significant body of indirect evidence on the associations of adherence to the Canadian 24 h movement guidelines by children and adolescents with a wide range of outcomes, which suggests that there may be broader health benefits.[121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140] The potential benefits range from improved fitness and reduced obesity, through to a range of improvements in mental health. The evidence is summarized in Table 3.

Table 3.

Associations of adherence to the Canadian 24 h activity guidelines

Higher levels of optimism and satisfaction with life, and reduction in depressive symptoms, particularly for girls[128]

Lower physical-mental co-morbidity[138]

Lower myopia prevalence (OR 0.4–0.5)[141]

Lower risk of being bullied or being a bully[124]

Better overall physical, cognitive, and social-emotional development[126]

Better self-rated physical and mental health[137]

Lower levels of adiposity, behavioural problems, depression, and low self-esteem[122]

Better school-readiness[139]

Lower levels of anxiety and depressive symptoms[131]

Lower levels of suicidality[130]

Lower levels of internalizing and externalizing behaviors[135]

Less involvement in bullying behaviors[132]

Lower levels of depressive symptoms and other mental health problems[134]

Better academic performance[127]

Less impulsive behavior[125]

Fewer emotional and behavioral problems[123]

Greater levels of global cognition[140]

Lower BMI[129]

In general, increased compliance with the guidelines was associated with better outcomes

OR=Odds ratio, BMI=Body mass index

The guidelines for infants, preschoolers, children, and adolescents cover time for physical activity, sleep time, and screen time, and are available online (https://csepguidelines.ca). The guidelines do not specify where physical activity should take place, and emphasizing outdoor time could provide greater health benefits. The proportion of children who adhere to these guidelines is internationally quite low.[128]

Adherence to these guidelines may be relevant to myopia because children who adhere to them are less likely to be myopic or have reduced visual acuity.[141,142] Physical activity and sleep time have been associated with myopia, as has excessive sedentary time and recreational screen time,[143] but the evidence on all is somewhat inconsistent, and causal pathways are not clear.[11] It is important to stress that at the moment, the evidence is based only on associations, and causality has not been demonstrated.

One of the very important observations is that adherence to these guidelines is associated with improved educational outcomes.[126] This observation aligns with results from the STORM study that students recording up to 2 h outdoors per day were achieving better examination results that those with less time outdoors, although the results of those with higher levels of time outdoors showed decreases in performance.[120] Similarly, a trial of 2 h of outdoor sport after school each day showed that this did not lead to a decrease in academic outcomes but did result in significant improvements in student physical fitness. In this trial, there was a slight, but not significant reduction in the prevalence of myopia. These results are consistent with modern educational philosophies which suggest that creative play, rather than intensive instruction, provides a better basis for developing resilience, life-long, and creative learners, and ultimately better educational outcomes. As increased time outdoors is implemented in schools, it will be important to investigate the possible benefits in more general student physical and mental health, as well as in educational outcomes.

The Challenges and Promises of Implementation

The evidence discussed above shows that increasing time outdoors can be used to slow the development of myopia, particularly at the preschool and primary school levels. The practical challenge is that most parts of East Asia and Singapore have developed education systems based on the early onset of educational pressures, with a heavy emphasis on formal learning from the preschool years, which is often paralleled by heavy homework loads. This pattern continues throughout the school years and is supplemented, again often from the preschool years on, by extensive use of private supplementary tutorial education. The systems are also characterized by a heavy emphasis on the development of selective academic streams and schools, where enrollment is highly competitive.[144]

For these reasons, there has been considerable reluctance to change the education system; from governments, because an educated population is seen as the basis for future economic prosperity, and from parents who desire educational success for their children. There are also reasons for governments to make changes beyond the issue of myopia, including concerns about student mental health, lack of resilience and burnout, and concerns about obesity and fitness.

So far, only limited steps have been taken to change the school curriculum. The most substantial steps taken so far are those in China, where President Xi made the control of myopia a national priority in 2018. This was followed by what is known as the Double Reductions policy in 2021, which aimed to reduce the load on both students and parents in the educational process.[145] The most spectacular element of this policy was the virtual abolition of the coaching school system through a requirement that they operate on a not-for-profit basis. However, there was an underlying emphasis on changes designed to minimize educational pressures in the early years through reductions in examinations and homework loads and reduce selective pathways in schools that promote competition for places. While these changes do not appear to have been driven by concerns about myopia, the policies include changes that should facilitate myopia prevention. These policies are still very much a work in progress, and it is important that changes in refraction be monitored as they are rolled out.

It is also important to consider the costs of any roll-out of preventive initiatives. Increasing time outdoors is a very low-cost option. However, increasing time outdoors is an option that creates concerns among parents worried about the impact on their child’s education, and this option needs to be implemented in a way that ensures that it applies to all children. In China, the current emphasis on full-service schools that provide supervision for students after the end of schooling until parental pick-up time would provide an ideal time for implementation, although no doubt there will also be a tendency to use this time for supervised homework.

An area where there has been little research up until now is on the issue of changes to the organization and curriculum of schools that can reduce myopiagenic pressures, as well as providing time for more time outdoors. This will require specialists in vision working closely with experts in education, on both the design of changes and the monitoring of outcomes. The possibilities range from exploration of the extent to which some classes can be delivered out of doors, to greater emphasis on learning through creative play rather than formal instruction in the early years (https://www.oecd.org › oecd › reports ›2019/03) The recent policy developments in China provide a promising environment for work of this kind.

Conclusions

The results examined in this review lead to some clear conclusions.

1. Increasing the amount of time that children spend outdoors slows the onset of myopia in children, and slowing the onset is likely to lead to disproportionate reductions in the prevalence of high myopia and its pathological consequences. This preventive approach can be effectively implemented through school-based public health interventions

2. Increased time outdoors may also slow the progression of myopia, based largely on the evidence of seasonal variations in progression rates. This justifies recommending increased time outdoors to families with myopic children, particularly if there are non-myopic siblings. However, at present, myopia control should be based on well-validated clinical myopia control interventions

3. Prevention of myopia needs to start in the early years, with interventions starting in the preschool years, and continuing into higher levels of education. Given that there is little myopia in preschools in many parts of the world, and that there is little change in mean spherical equivalent refraction (SER) as well, efficacy may need to be monitored by changes in surrogate measures such as the axial length (AL)/corneal radius ratio (CR) ratio

4. As increased time outdoors is increasingly put into practice, it is important to monitor changes in educational outcomes and in a broad range of physical and psychosocial outcomes

5. The light-dopamine hypothesis has been validated as one of the mechanisms mediating the causal impact of time outdoors on the development of myopia. This does not preclude other mechanisms playing a role, and in particular, synergistic interactions between increased time outdoors and changing the classroom environment through brighter lighting or outdoor scene classrooms may also produce powerful effects.

The main challenge for the future is how to overcome the barriers to implementation. Expanded research on how to decrease myopiagenic pressures in school, without compromising educational outcomes, is therefore an important priority, as is assessing the broader outcomes associated with increasing time outdoors in schools.

Data availability statement

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

Conflicts of interest

The authors declare that there are no conflicts of interest of this paper.

Funding Statement

Nil.