Ten years ago, myopia affected approximately 1.5 billion individuals worldwide, with a prevalence rate of 22% (1). The worldwide surge in myopia has led to a projected estimation of 4.7 billion individuals by 2050 (2). Myopia has emerged as a global public health concern in recent years, especially in China and other East Asian countries. In China, the prevalence of myopia among children aged 7–12 increased from 25.3% in 2008 to 32.8% in 2022 (3), with the highest incidence noted among primary school children (4).

Ocular refraction is dependent on optical and biometric parameters of the eye including curvatures of anterior and posterior corneal and lenticular surfaces, axial thicknesses of cornea, anterior chamber, lens and vitreous chamber, and refractive indices of ocular media (cornea, aqueous, lens and vitreous). Predominantly, change in ocular refraction occurs during the early childhood which is primarily brought about by the coordinated growth of its refractive components, including corneal and lens refractive power (Km, LP), anterior chamber depth (ACD), lens thickness (LT), and axial length (AL) (5). The changes in ocular refraction may also be influenced by eye use behaviors and environments such as exposure to outdoor illumination. As the epidemiology of myopia has increasingly focused on risk factors (6), researchers have identified increased near work and/or decreased time spent outdoors as possible factors contributing to its onset (7, 8). The majority of the previous studies investigating myopia-related environmental factors quantified through questionnaires, often leading to inaccurate information due to recall bias (9). More recently, investigators have used technologically more advanced electronic devices to monitor eye use behaviors objectively. The Clouclip M2 is one such wearable device that monitors the eye use behavior and its environment, such as durations and distances of near work, time spent on outdoor activity, and luminance of the working environment. Since accurate usage characteristics are vital to arrive at proper conclusions, and objective method like this could provide more reliable parameters that subjective questionnairs which are dependent on memory, cannot provide.

This study used both objective (using Clouclip M2 device) and subjective (using survey questionnaire) methods to determine eye use behavior and visual environment in children with a view to assess if these differ between myopic and non-myopic individuals. The results would be worthwhile in identifying the factors influencing myopia development and provide a theoretical basis for prevention and control of myopia. Additionally, we also investigated the relationship between optical parameters of the eye (AL, Km, LT, ACD, etc.) and refractive status.

The development and implementation of this study was approved by the Ethics Committee of Aier Eye Hospital, Shijiazhuang, and all procedures followed the principles of the Declaration of Helsinki. Informed consents were sought and obtained from the students and their parents voluntarily prior to administering study related eye examinations. Clouclip M2 monitoring device was issued after obtaining informed consent from the school principal, homeroom teacher, along with students, and parents.

Students of Caochang Street Primary School (Shijiazhuang, Hebei province, China) were screened for eligibility. The inclusion criteria were age between 9 and 11 years, willing to consent to participate, willing to wear the device during the entire study period, be able to standardizing Clouclip M2 usage, no apparent abnormalities; be able to respond survey questionnaire and attend follow-up examinations.

Students with previous history of amblyopia, eye trauma, or eye pathology other than refractive error, general or mental illness, undergoing local or systemic drug use affecting test results, not sleeping between 24:00 and 6:00, undergoing orthokeratology, low-concentration atropine eye drops, repeated low-level red-light, or visual training, and unable or unwilling to comply with the Clouclip wearing requirements were excluded from the study. Data from the students not responding to survey questionnaire, not returning the Clouclip prior to 7 days of wear and not returning for eye evaluations on 8th day were not included in the analysis.

A total 257 primary school students aged between 9 and 11 years screened and consented to participate in the study. All participants were instructed to wear Clouclip M2 for 7 consecutive days, including 5 working days and 2 rest days. On the 8th day, accompanied by their parents, the students returned the Clouclip M2 and underwent relevant eye examinations and completed the survey questionnaire. Data from Clouclip M2 was then retrieved and analysed.

Among the 257 students, 39 had incomplete data collected by the clouclip M2, 4 could not cooperate to complete the questionnaire, and 2 withdrew from the study midway. Remaining 212 (105 boys and 107 girls) were included in the statistical analysis in final.

Data from a total of 212 students (105 males and 107 females) were analysed, out of which, 162 students were myopic, 47 were emmetropic and 3 (only females) were hyperopic (Table 1).

Ocular structures continue to develop during early childhood. Emmetropization is the process of precise and coordinated changes in refractive components of the eye to achieve an optical perfection; and a derailed development may result in refractive errors. The refractive error results from a multifactorial condition involving a complex interplay between the cornea, the lens and the length of the eye (11, 12). Children aged 7–12 years old are in an essential stage of myopia occurrence and development (13), making them the key population in the prevention and control of myopia. Apart from the widely acknowledged genetic disposition, eye use behavior and visual environmental influence have also been frequently reported as the causes of myopia. Number of studies have provided evidence-based recommendations of an appropriate eye use behaviors for children and adolescents in an attempt to reduce occurance of myopia (14).

This study utilized three-fold investigation methods; first, it examined the relationships between refractive status of eye with its biometric parameters; second, it looked at the correlation between objective assessment vs. the subjective reporting of the study parameters; and third, it evaluated whether eye use behavior and environmental factors were associated with myopia. While survey questionnaire have been the primary means of gathering information on eye use behavior traditionally, recently, technologically advanced electronic devices are commercially available which are more efficient and accurate in data collection. Wearable sensors are increasingly used in continuously monitoring and recording the eye use behavior objectively (15). Clouclip M2 is found to be more accurate in determining the light level into the eye (16) compared to other available devices such as Actiwatch (17), Fitsight (18) and HOBO (19). The Clouclip records eye use duration and distance in real-time and continuously processes the data (20). Moreover, Clouclip not only measures the viewing distance but also records luminance (18). Number of studies have convincingly claimed that Clouclip has a good practicability, is accurate and has a good stability in the objective measurement of working distance, eye use time and luminance, for which, it is highly recommended for myopia related research studies (21, 22).

It is worth noting that, in previous studies, the correlation between SE and ACD has been inconsistent. For instance, consistent with the result of Hosny (22), Wang et al. (24) found a negative correlation (r = −0.623, P < 0.01), between ACD and SE in children aged 5–12 years in Lanzhou (r = −0.498, P < 0.01), while Zhou et al. (25) reported no significant correlation in children aged 3–14 years. Such varying results may be related to the differences in race, region, age range, refractive status, and ocular biometrics of the subjects studied. AL and CR are the two primary parameters governing the refractive status of the eye. The ratio of axial length to corneal curvature (AL/CR), first proposed by Grosvenor (26), has been frequently used in predicting onset and monitoring myopia progression where cycloplegic refraction is not available (27). The AL/CR > 3.0 is proposed to indicate myopia (28). This ratio, being an objective measure, is relatively more reliable for less interference by subjective and regulatory factors. Consistently, our results showed that SE was highly correlated with AL/CR (r = −0.859, P < 0.01).

Compared to the subjective estimate using survey questionnaire, the Clouclip M2 measurements yielded significantly longer average total time for near work per day, the maximum time for single near work and the average distance of near work (P ≤ 0.001), and significantly shorter average total time for outdoor activities per day (P < 0.001). There are two possible reasons for this difference. Firstly, the questionnaire survey is influenced by the respondents’ subjective estimate and memory. Parents are likely to believe that myopia is caused by individual factors such as improper eye posture, and therefore may have responded with their pre-meditated “mindset”. This was evident in our result where study time was negatively associated with outdoor activity time. The study time of students in this study primarily determined by the homework set by teachers and extra study tasks set by parents. Secondly, parents and students needed to remember the time while responding the questionnaires with possibility of recall bias. Study time, outdoor time, and near work distance are mere estimates, and such bias is inevitable. Clouclip device allows a real-time recording objectively potentially eliminating the subjective bias. Accuracy in reported time may have significant implications in interpreting the results.

Duration of near-work and shorter working distance have been associated with increased risk of myopia development and progression (29, 30). However, whether these have a casual effect or this is by a mere chance is debated. A study highlighted that children with myopia tend to engage in more near work than children without myopia (31) whereas another study found no or limited role of near work as pathogenesis of myopia (32). According to a meta-analysis, 10 out of 15 cross-sectional studies found that increased prevalence of myopia is associated with longer near-work activities (32). In this study, Clouclip measured near working distance of 32.21 cm which is slightly further than that determined subjectively using survey questionnaire (24.94 cm) but is consistent with a previous report (32 cm) for among 15 years old (33). The observed differences may be linked to variations in data sources and collection method. Previous studies have shown that students spending more than 3 h a day in close work had significantly increased AL compared to those spending less than 3 h (35). Shorter near working distance (<30 cm) and longer reading time (>30 min) increased the risk of myopia development by 2.5 times and 1.5 times (30), respectively suggesting that too long near work and too close working distance are risk factors of myopia. Considering 180 min of daily near working time, 60 min of engagement in a single near task and 30 cm of working distance as cut of values for clouclip (15, 30, 34, 35). Our results, both for Cluclip and questionnaire data, consistently found that myopia was associated with the longer time spent for near work which was evident in the result of our binary multivariate analysis. The results of questionnaire survey and clouclip showed that sustained proximity time and working distance had an effect on the incidence of myopia but were not risk factors for myopia. 30 cm of working distance was not associated with myopia as the results of clouclip.

Some earlier studies have shown that the average outdoor time measured by Clouclip was 24 min per day on weekdays and 54 min per day on rest days (36), which was lower than the value in this study (61.50 min). The difference may be attributed to the variation in geographic regions, season in which the study was conducted and student articipants’ learning intensity. Previous studies have suggested that increasing outdoor activities may effectively reduce myopia occurrence in school-age children (31, 37). Results of this study, obtained from both survey questionnaire and Clouclip, consistently showed that children spending outdoor for more than 90 min per day were significantly less likely to develop myopia. Our results suggest that students with less than 200 lux daytime luminance were 2.04 times more likely to develop myopia which supports previous proposal of outdoor activity duration and luminance influencing myopia (38). These results may support earlier hypothesis that exposure to higher-intensity daylight may prevent the onset of myopia in children (19, 39).

An item in our survey questionnaire contained information on whether they use eye protection lamp as a measure to for myopia control. The results revealed that the proportion of myopia was significantly higher among those students using eye protection lamps. This finding contradicts a general belief that the lamp should have controlled myopia. Although we did not ask whether they started using the protection lamp prior to or after the onset of myopia, the unexpected finding could be related to the fact that parents of already myopic students were more concerned to their children's refractive error and they replaced their standard light with the lamp in an attempt to arrest myopia from further progression. Further investigation is required to accurately determine the protective or causative effect of the protection lamp on myopia.

Parental myopia is widely known to influence myopia occurrence in children. A previous study from Qingdao, China, found that children with both myopic parents were 2.58 times more likely to develop myopia compared to those without myopic parents (40). Consistent with the report, we found that children with one myopic parent were 2.86 times likely to become myopic. Children who have both myopic parents are 3.80 times higher chance of becoming myopic compared to those without any myopic parent. These results further supports the arguably undeniable role of genetic factors in the incidence of myopia in children.

Our study has some limitations. Although the Clouclip M2 measures luminance with a reasonable accuracy, it does not detect the type of light source (e.g., natural light vs. electric light). Also, it does not identify whether the wearer is looking at paper surface or monitor of an electronic device. Although it is yet to be determined whether above mentioned factors have any influence on myopia development, it would be desirable to investigate in the future. Therefore, studies of this nature should use an appropriate questionnaire tool along with the device to monitor associated factors more accurately. Further, our data suggest that myopic children were more compliant with study procedure and wore the device in a standardized manner compared to non-myopic children during the study. However, the difference in compliance was practically insignificant; therefore, we believe that the difference would have none to minimal influence in the overall outcome of this study. The relatively smaller sample size of non-myopic students as compared myopic sample is another limitation of the study. Future study with more balanced sample size is desirable for better credibility of the results.

In conclusion, our study further supports the proposal of eye-using behavior, eye-using environment, and parents’ myopia are strongly associated in the occurrence and development of myopia in children. Clouclip M2 provides valuable information on eye use behavior and visual environment which can be used in investigating myopia related studies. Addition of more elaborative items in survey questionnaires in relation to specific eye use behaviors may be useful in better understanding of the effect of environmental factors on development of myopia.