In this study we found two significant associations between visual activity and refractive status: (1) Myopes spent significantly fewer hours engaged in sports/outdoor activities than non-myopes during the school year but not in the summer, and (2) myopes spent significantly more hours watching TV than non-myopes during the school year but not in the summer.
Our results for sports/outdoor activity in the school year agree with previously published findings.6–7,17,20–21
The Sydney myopia study demonstrated a highly significant association between outdoor activity and the refraction of children aged 6 and 12 years.7
Likewise, a longitudinal study conducted in the U.S. reported that children spending more hours in sports/outdoor activity starting from 6 years had a reduced risk of developing myopia by the age of 13 years.17
An exploratory data analysis of outdoor activity hours using longitudinal refraction data from our laboratory showed a similar trend.
An additional finding from Jones et al.17
was that 14 or more hours of weekly sports/outdoor activities essentially negated the impact of parental myopia. However, a high amount of outdoor activity time may not be able to completely protect against the development and progression of myopia. A recent study reported an average of more than 20 hours of weekly outdoor activity among both non-myopic and myopic teenagers in Singapore, a country with a very high prevalence of myopia.21
One wonders if most of this outdoor time was during the evening after the daylight hours.
Given the limited time available for all types of visual activities in a child’s day, it is possible that the significant association between sports/outdoor activity and refractive status might reflect a trade-off between this activity and another indoor activity such as computer use or TV viewing. However, in our study there were no significant negative correlations, and only correlations between hours spent on sports/outdoor activity and TV watching approached significance and only in the myopes. It appears, therefore, that the association of sports/outdoor activity and refractive status is unlikely to be caused by the substitution of sports/outdoor activity with other activities.
The finding of a positive association between TV watching and refractive status in our study has not been found in most other studies. In the Orinda study hours spent on TV viewing were slightly higher for myopic children.6
A recent study from Taiwan reported finding a borderline significant correlation between myopia and television watching.26
As evidence for TV watching-induced myopia is lacking in the literature, further studies are needed to confirm the association reported here.
Although overall myopes spent significantly more hours than non-myopes in reading for pleasure during the summer break, this difference is mainly driven by one age group (10–14 years) and is not robust since the odds ratio associated with reading is not significantly higher than 1.
An advantage of our study is that it examined activity hours in both school and summer periods. Low correlations between the school year and the summer break for both studying and sports/outdoor activity indicate that these activities may depend on the school schedule. High numbers of summer sports/outdoor activity hours were noted for both myopes and non-myopes, suggesting that outdoor activities may slow eye growth in all children during the summer break. In agreement with this view, slowed myopia progression during the summer break27
and a positive association between more outdoor activity time and less myopic progression during the vacation period have been reported.28
Another advantage of our study is that the wide age range provides an opportunity to explore the pattern of age dependence of various activities. For instance, the monotonic increase in study time with age was clearly demonstrated in our data. The recognition of age dependence allows for a more meaningful comparison of studies with different age cohorts.
Reported visual activity times have varied by studies, as shown in . Possible sources of variability may include survey age, regional differences, ethnicity, and the design of the questionnaire. Despite these differences, most of the visual activity times reported in our study were comparable to results published previously. As seen in the table, studies conducted in Singapore and Australia had a larger range of outdoor activity hours than those conducted in the U.S. The number of hours spent in sports/outdoor activity in our study for the school year was similar to that reported for the Orinda study.6,17
The summer weekly sports/outdoor activity time in our study was close to the number given in a recent study in Singapore.21
Comparison of weekly activity time (mean±SD) evaluated by questionnaire across studies.
It is not clear which aspects of outdoor activity provide a protective effect, but it appears to be related to being outdoors and not the nature of the activity. Two studies fine-tuned their questionnaire and evaluated both outdoor and indoor sports activity. Sports activity was not the key factor since indoor activity hours did not show a difference between myopes and non-myopes.7,21
Suggestions for the source of the protective effect of being outdoors include pupillary constriction, increased dopamine levels, and distance viewing. One possibility is that the contraction of the pupil in sunlight increases the depth of focus, decreases blurriness, and thus slows eye growth. More exposure to sunlight may also increase the amount of dopamine, an inhibitor of axial elongation. A third possibility focuses on distance viewing, which may provide a stop signal for eye growth.
The current study had some limitations related to the sample size and age range of the participants. Because of the relatively small number of myopes in our study, the conclusions may be more sensitive to sampling errors. The survey age range used in the analyses was broad (12 years), similar to the range in a Singapore study21
(9 years). Although statistical adjustment was made to account for the age difference at the time of the survey, the adjustment relies on the age dependence pattern observed in our sample. The conclusions could be different in other studies if the relationship between survey age and activity time differs.
In conclusion, we found more hours of sports/outdoor activity among non-myopic compared to myopic children in the school year but not in the summer break. In agreement with other studies, the non-myopes had a higher number of weekly sports/outdoor activity hours during the school year, which may protect against myopia development. A new finding is the high number of sports/outdoor activity hours for both myopes and non-myopes during the summer break, which may contribute to slowed eye growth in all children during these three months.