The study was conducted on a representative sample of Czech adolescents aged 11–16. It was a longitudinal study, meaning we followed the same group of adolescents and collected data from them three times over the course of a year. The type of analysis we used allowed us to see whether the links between screen time and sleep reflect general differences between people – for example, those who naturally stay up late also tend to spend more time on screens – or whether they show actual changes within the same person over time. In other words, we were able to see whether screen use leads to changes in sleep later, or whether it is actually the sleep issues that lead to more screen time.
Stable differences between adolescents
Our study showed that adolescents who feel sleepier during the day also spend more time on screens and go to bed later than their less sleepy peers. However, these individual differences don’t necessarily prove cause and effect. They simply show that these habits often go hand-in-hand. They can be explained by other shared factors that influence all three areas (screen use, bedtime, daily sleepiness) at once. One such factor might be the circadian rhythm. In simple terms, it is about whether an adolescent is an "early bird" or a "night owl." Another possible common cause could be different parenting styles. For instance, adolescents whose parents don't set a firm bedtime have more free time in the evening. They can spend it on digital media. For these adolescents, going to bed late combined with waking up early for school can lead to more daytime sleepiness.
The reinforcing loop between screen time and later bedtime
The study results further showed changes in media use. Specifically, deviations from usual screen time were associated with changes in bedtimes in two ways.
First, increases in screen time and later bedtimes were associated during the same periods of measurement. This pattern probably reflects two realities: screen time and sleep compete for time in the daily schedule, and later bedtimes mean more free time, which can be filled with media use, for instance.
Second, between the periods studied, the relationship between screen use and bedtime was mutual and took the form of a reinforcing loop. When adolescents spent more time than usual in front of screens in one period, they went to bed later in the following period. This later bedtime was then associated with a further increase in media use. The pattern we found suggests that screen time has a negative effect on bedtime in adolescents, but it was not consistent across all studied periods.
In the case of daytime sleepiness, no similar association over time was found. However, it is possible that more screen time is associated with higher daytime sleepiness only when adolescents go to bed later as a result. Our study did not directly test this type of relationship, and it remains a subject for further research.
Limiting screen time before bed: does it work?
Our study also focused on whether limiting screen use in the last hour before bedtime, which is commonly recommended as part of sleep hygiene (e.g., by the National Sleep Foundation), mitigates the potential negative effects of media use on adolescents‘ sleep. However, the study found no evidence of the long-term effectiveness of reducing screen use before sleep. Specifically, the relationships found between changes in media use and sleep did not differ between adolescents who limited their screen use before bedtime in the first observation period and those who did not. The instability of screen restriction over time may partly explain why its expected protective effect on sleep did not manifest itself in the longer term.
Beyond these findings, adolescents who do not use screens in the last hour before bedtime tend to spend less time with media, go to bed earlier, and experience less daytime sleepiness. However, these differences likely reflect stable characteristics of these adolescents, such as greater self-regulation or characteristics of their family environment, such as more restrictive parenting.
Important message for parents: It is not the amount of time spent in front of screens that plays a key role in adolescents‘ sleep, but rather the regularity and timing of sleep. Our findings also showed that late bedtimes tend to keep getting even later over time. This trend may be related to biological changes during adolescence, as well as to increasing autonomy or school demands.
Later bedtimes are linked to both higher daytime sleepiness and a subsequent increase in screen time. Therefore, strategies focused on earlier and more consistent bedtimes and supportive family routines may be more effective for a child’s sleep health than simply trying to limit screen use on its own.
