Objective: To assess the prevalence of the use of mobile phones by adolescents after lights out and its relationship to tiredness levels after one year.
Design: Prospective cohort study with self-reports and follow up questionnaire after one year.
Setting: Second- and fifth-year secondary school children in 15 schools in Flanders, Belgium.
Participants: 1656 school children; 52.1% boys. Average age was 13.7 years (SD: 0.68) in the youngest group and 16.9 years (SD: 0.83) in the oldest group at baseline.
Main outcome measures: Self-reported tiredness.
Results: Only 38% of the subjects never used their mobile phone after lights out. Multinomial logistic regression showed that using the mobile phone less than once a month increased the odds of being very tired one year later by 1.8 (95% CI 1.2-2.8). Those who used it less than once a week were 2.2 times more likely to be very tired (95% CI 1.4-3.5). Using it about once a week increased the odds by 3.3 (95% CI 1.9-5.7) and those who used it more than once a week were 5.1 times more likely to be very tired (95% CI 2.5-10.4). Overall 35% of the cases of being very tired were attributed to the use of the mobile phone. Use of the phone right after lights out increased the odds of being very tired by 2.2 (95% CI 1.4-3.4); between 00:00 and 03:00 the odds were 3.9 times higher (95% CI 2.1-7.1), and in those who used it at any time of the night the odds were 3.3 times higher (95% CI 1.8-6.0).
Conclusion: Mobile phone use after lights out is very prevalent among adolescents. Its use is related to increased levels of tiredness. There is no safe dose and no safe time for using the mobile phone for text messaging or for calling after lights out.