Objectives: Apple's iPhone Night Shift feature purports to reduce short-wavelength light emissions and improve sleep. We aimed to investigate these claims by comparing emerging adults' sleep outcomes associated with smartphone use before bed with iPhone's Night Shift enabled to two comparison conditions (iPhone use with no Night Shift, no iPhone use).

Design: Participants were randomly assigned to one of three conditions specifying iPhone use during the hour preceding bedtime for seven consecutive nights: iPhone use with Night Shift enabled; iPhone use with Night Shift disabled; and no phone use.

Setting: Participants were recruited from a western undergraduate university.

Participants: A sample of 167 emerging adults (ages 18-24; 71.3% female) with iPhones participated in the study.

Measurements: Sleep outcomes (sleep latency, duration, efficiency and wake after sleep onset) were tracked using wrist-worn accelerometers.

Results: There were no significant differences in sleep outcomes across the three experimental groups. Post-hoc exploratory stratified analyses revealed a significant main effect of phone condition on sleep efficiency (P = .014) and WASO (P = .013) for participants averaging more than 6.8 hours of sleep per night, with the no phone condition demonstrating the best sleep outcomes. For those averaging less than 6.8 hours of sleep, there was no effect of phone condition on sleep outcomes.

Conclusions: Across our full study sample, there were no differences in sleep outcomes attributable to Night Shift. For individuals who regularly obtained adequate sleep, abstaining from screen use resulted in better quality sleep than did phone use with Night Shift enabled.