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). 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).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).

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