BACKGROUND: Round-the-clock use of smartphones holds a potential for awakenings and/or shorter sleep duration, which may have adverse health consequences. We aim to describe overnight smartphone activity among young adults and to characterize those with smartphone interrupted sleep in terms of sleep impairment and mental and physical health indicators.

METHODS: We use unique objective high-resolution information on timing of smartphone activity (based on >250,000 phone actions) continuously monitored over a four-week period among 815 young adults combined with indicators of mental and physical health.

RESULTS: We find substantial overnight smartphone activity. More than 12% had smartphone activity in the middle of the night (3 to 5 hours after self-reported bedtime) and 41% had smartphone interrupted sleep on at least one weekday during a 4-week period. Those with frequent smartphone interrupted sleep had on average 48 minutes shorter self-reported sleep duration and higher body mass index, whereas there were no differences in physical or mental health symptoms.

CONCLUSIONS: The substantial smartphone activity during bed hours among young adults may pose a public health challenge and especially the relation to overweight warrants close attention.