Table 1 Measured indicators and definitions

Frequency of digital use (FDU)

The substitute variable for frequency of digital use considered 1) online duration of mobile devices (scale: 0–18); 2) online duration of computer use (scale: 0–10); 3) whether to use online computer (1 = yes, 0 = no).

Types of digital use (TDU)

Digital entertainment

(DE)

The substitute variable for digital entertainment considered 1) whether to play online games (1 = yes, 0 = no); 2) frequency of playing online games (never, 0; not every day, 1; daily, 2); 3) whether to watch a short video (1 = yes, 0 = no); 4) frequency of watching the short video (never, 0; not every day, 1; daily, 2); 5) the importance of the Internet to leisure and entertainment (scale: 1–5).

Digital consumption

(DC)

The substitute variable for digital consumption considered 1) whether to shop online (1 = yes, 0 = no); 2) frequency of online shopping (never, 0; not every day, 1; daily, 2); 3) the importance of the Internet in daily life (scale: 1–5).

Digital social interaction

(DI)

The substitute variable for digital social interaction considered 1) whether to use WeChat (1 = yes, 0 = no); 2) frequency of sharing moments with friends online (never, 0; not every day, 1; daily, 2); 3) the importance of the Internet for social interaction (scale: 1–5).

Digital learning

(DL)

The substitute variable for digital learning considered 1) whether to learn online every day (1 = yes, 0 = no); 2) frequency of online learning (never, 0; not every day, 1; daily,2); 3) the importance of online learning (scale: 1–5).

Diet

The substitute variable for diet considered 1) whether to consume protein in the past week (1 = yes, 0 = no); 2) whether to consume fruits and vegetables in the past week (1 = yes, 0 = no). The closer to 1, the healthier.

Sleep

The substitute variable for sleep considered 1) sleep duration (sleep ≤ 5 h or sleep ≥ 9 h, 0; 5 h<sleep<9 h, 1); 2) whether to take a afternoon nap (1 = yes, 0 = no). The closer to 1, the healthier.

Exercise

The substitute variable for exercise considered 1) exercise frequency (never, 1; less than once per month, 2; more than once per month but less than once per week, 3; 1–2 times per week, 4; 3–4 times per week, 5; more than 5 times per week, 6); 2) exercise duration (0, 0 min; 1, 1-60 min; 2 61-120 min; 3, duration over120min); 3) exercise feeling (1, feeling bad; 2, feeling ordinary; 3, feeling good). The higher the number, the healthier.

Smoking and drinking

The substitute variable for smoking and drinking considered 1) number of cigarettes smoked per day (0, do not smoke; 1, smoke 1–20; 2, smoke 21–40; 3, smoke over 40); 2) whether to smoke in the past month (1 = yes, 0 = no); 3) whether to drink alcohol more than three times in the past (1 = yes, 0 = no). The lower the number, the healthier.

Depression

The depression variable was measured by the Center for the Epidemiological Studies Depression Scale (CES-D), which comprises of eight items: 1) I feel depressed; 2) I find it difficult to do anything; 3) My sleep is not good; 4) I feel happy; 5) I experience a sense of solitude; 6) I have a happy life; 7) I feel sad; 8) I do not think life can go on (1, never; 2, sometimes; 3, often; 4, always). The lower the number, the healthier.

Gender

0, female; 1, male

Age

1, 60–69 years; 2, 70–79 years; 3, 80 years and older

Area

0, rural area; 1, urban area

Participation on medical insurance

1 = yes, 0 = no

Education

0, uneducated; 1, educated (at least primary school)