Havva Sert’s research while affiliated with Süleyman Demirel University and other places

... Excessive use of smartphone (defined as 5 or more hours during a weekday) emerged as the most powerful independent positive predictor of smartphone addiction controlling for age at first smartphone use, age, personality type, depression and anxiety scores, class, and purpose of using smartphone. This is in agreement with prior reports of a significant positive association between amount of time spent daily using smartphones (>16 vs. < 4 hours) and smartphone addiction score in Turkish medical students (Demirci et al., 2014), and amount of time spent using the Internet per day and smartphone addiction score in Korean college students . ...

Reference:

Is Smartphone Addiction Associated with a Younger Age at First Use in University Students?