The aim of this study is to examine the relationship among smartphone addiction, fear of missing out (FoMO), excessive use of social networking sites (SNSs) on smartphones and sleep duration among high school students. The study group consists of 161 high school students from two state high schools in Turkey. Pearson correlation coefficient and stepwise multiple regression analysis techniques were used in the study, designed according to the cross-sectional method. Based on the findings of the study, there are positively correlated relationships between age, daily duration of SNS use on Smartphone, duration of smartphone ownership, and FoMO variables and smartphone addiction. In addition, there are negatively correlated relationships between duration of sleep (DoS), checking frequency of SNS (CFoS) on smartphone variables and smartphone addiction. Finally, FoMO, daily duration of SNS use on smartphone and duration of smartphone ownership predict smartphone addiction. The FoMO variable is the strongest predictor of smartphone addiction among these variables.