After exclusion of missing records and inconsistencies, 581 children (291 [50%] boys) participated at the age of 4 years. At the ages of 5 and 6 years, data were available for 508 participants (259 [51%] boys) and 426 participants (211 [50%] boys), respectively (a flowchart of participant selection is reported in Supplementary Fig. 1). The median age of the mothers was 33 years (range 21–54 years), approximately 70% had graduated from high school, and 11% of the mothers had an independent or high-profile/leadership job (Fig. 1).

Time spent with digital media use and book reading (means and 95th centile, asterisks indicate statistically significant differences)

Most time spent with digital media use was represented by time spent with passive activities in front of a screen. We also observed that time spent with digital media use increased with child age. There were no substantial differences between boys and girls at the ages of 4 and 5 years. In contrast, we found a statistically significant difference at the age of 6 years with girls spending 30 min/day more compared to boys on a weekday and 75 min/day more compared to boys on weekends. Moreover, we observed a larger use of digital media at weekends compared to weekdays in both boys and girls, with a difference of 20, 24, and 54 min at the ages of 4, 5, and 6 years, respectively. In boys, these differences were 20, 25, and 31 min, whereas the values were higher in girls, especially at age 6 years, with 21, 33, and 76 min, respectively. We also detected that a comparable amount of time was spent with “books” when compared to “all digital media” at ages 4 and 5 years, both for boys and girls and during weekdays and at weekends. This shifted at age 6 years, when more time at the weekend was spent with “all digital media” compared to “books,” especially in girls. The time spent on different media activities between 4 and 6 years of age is reported in Fig. 1 for boys and girls separately. Investigating the timing across the day, time spent passively with “TV/screen” and on “book reading” had a similar pattern, with less time spent on these activities in the morning compared to the evening (Supplementary Fig. 2).

On the contrary, more time was spent with “gaming” during the afternoon. No particular patterns were observed at different ages, or by sex, or during weekdays and at weekends, apart from a higher use of “all digital media” and “books” in the mornings of weekends (Fig. 2, Supplementary Fig. 3).

Percentages of time spent with different activities during morning, afternoon, and evening (all digital media and book reading)

Concerning sleep problems, a similar pattern of frequency of fears and tendency to wake up at night was found for boys and girls at 4 years of age. In particular, at 4 years of age, 40% of boys and 38% of girls usually need their parents to fall asleep. About 10 and 13% of boys and girls usually feared the dark or feared sleeping alone, respectively. Finally, 26% of the boys and 24% of the girls usually woke up at night. The need for parents in the room to fall asleep decreased with age in both boys and girls. At 6 years of age, 24% of the boys and 23% of the girls needed their parents to fall asleep. The frequency of waking up at night also decreased over time, down to 16% of the boys and 10% of the girls usually waking up at 6 years of age. In contrast, fears increased with age, with 18% of the boys and 20% of the girls usually having a fear of the dark at the age of 6 years. The distribution of items regarding sleep-related fears is reported in Fig. 3.

Distribution of items on sleep-related fears from the Children’s Sleep Habits Questionnaire

The correspondence analyses of “books” and “all digital media” resulted in two well-defined dimensions. The first dimension separates high exposure to “all digital media” and low exposure to “books” from low exposure to “all digital media” and high exposure to “books.” This first dimension results in an explained variance of 53.5%, 50.2%, and 50.0% at the ages of 4, 5, and 6 years, respectively. The second dimension separates low exposure from high exposures to both “all digital media” and “books,” explaining the remaining 46.5%, 49.8%, and 50% of variance at the age of 4, 5, and 6 years, respectively. The three sleep-related fears (fear of the dark, fear of sleeping alone, and need of parent to fall asleep) cluster together at 4 years of age. They were related to higher exposure to “all digital media” and were apparently more common in girls. At the ages of 5 and 6 years, these relations are maintained for the fear of sleeping alone and fear of the dark. On the contrary, the need of parents to fall asleep appears to be a more independent factor at the ages of 5 and 6 years. The correspondence analysis also showed that waking up during night was more common in boys, irrespective of age. Notably, this sleep problem appears to be related to higher exposure to “books” at age 4, while at 5 and 6 years of age, it is related to lower exposure to “books.” Results of correspondence analysis of digital media use and book reading in association with sleep-related fears are reported in Fig. 4.

Correspondence analysis of media use (all media) and book reading: λ1‑2 percentage of explained variance by the first and second dimension, respectively; dotted arrows directions related to more/less use of digital media or book reading; ellipses clustering of sleep-related fear items and their correspondence with children’s sex; PFA need of parent to fall asleep; FSA fear of sleeping alone; FOD fear of the dark; WUN waking up in the night

Finally, the results from the GEE analyses adjusted for sex documented a statistically significantly increased risk of the fear of sleeping alone (relative risk [RR]: 1.35, [95% CI: 1.07; 1.70]) and fear of the dark (RR: 1.47 [95% CI 1.16; 1.87]) for exposure to “all digital media” higher than the median as compared to lower levels. Exposure to “books” above the median and compared to lower levels was tentatively associated with a relevant 18% reduction in the risk of both fear of sleeping alone and fear of the dark (RR: 0.82 [95% CI 0.65; 1.03] and RR: 0.82 [95% CI 0.64; 1.05], respectively). Similar results were obtained when the data were analyzed stratified by sex, with even stronger risk increases in boys (RR: 1.73 [95% CI 1.25; 2.38] and RR: 1.69 [95% CI 1.20; 2.39] for the fear of sleeping alone and fear of the dark, respectively).