| Smartphone usage time (year)1) | 7.5±2.6 | 7.2±2.5 | 0.140 | 1.02 (0.99–1.05) |
| Purpose of use | | | | |
| Social netwwork | | | | |
| No | 14 (43.7) | 18 (56.3) | | 1 |
| Yes | 326 (47.5) | 360 (52.5) | 0.676 | 1.09 (0.73–1.62) |
| Study | | | | |
| No | 23 (56.1) | 18 (43.9) | | 1 |
| Yes | 317 (46.8) | 360 (53.2) | 0.248 | 0.83 (0.63–1.11) |
| Entertainment | | | | |
| No | 25 (53.2) | 22 (46.8) | | 1 |
| Yes | 315 (46.9) | 356 (53.1) | 0.407 | 0.88 (0.67–1.17) |
| Basic calling, texting | | | | |
| No | 37 (48.1) | 40 (51.9) | | 1 |
| Yes | 303 (47.3) | 338 (52.8) | 0.897 | 0.98 (0.77–1.26) |
| Average smartphone usage in a day (hr) | | | | |
| <5 hr | 52 (33.3) | 104 (66.7) | <0.001 | 1 |
| ≥5 hr | 288 (51.3) | 274 (48.7) | 1.54 (1.21–1.95) |
| Use smartphone after waking up | | | | |
| No | 2 (40.0) | 3 (60.0) | 0.550 | 1 |
| Yes | 338 (47.4) | 375 (52.6) | 1.18 (0.40–3.48) |
| Average use time after waking up | | | | |
| ≤30 min | 259 (48.3) | 277 (51.7) | 0.090 | 1 |
| >30 min | 37 (59.7) | 25 (40.3) | 1.24 (0.99–1.54) |
| Use smartphone before sleeping | | | | |
| No | 10 (25.6) | 29 (74.4) | 0.005 | 1 |
| Yes | 329 (48.5) | 349 (51.5) | 1.89 (1.10–3.25) |
| Average time used before sleeping | | | | |
| ≤30 min | 127 (40.2) | 189 (59.8) | <0.001 | 1 |
| >30 min | 203 (56.1) | 159 (43.9) | 1.40 (1.19–1.64) |