Technological development in recent times has dramatically changed the way people live, interact with, and consume information. Since the emergence of the first iPhone in January 2007 until today, mobile phones are used daily for a range of purposes. Using mobile phones for various purposes intensified during the COVID-19 pandemic due to isolation or government lockdown regulations. However, along with the advantages of smartphone usage there are many disadvantages such as smartphone addiction and continuous exposure to digital screens, behaviors known as PSU—problematic smartphone use. This study explores the impact of several variables on PSU: loneliness, the need for social interaction, sleep hours, fear of losing phone access (nomophobia), and stress among young adults in Israel. The variables are examined with respect to two time periods: 1. During the COVID-19 pandemic lockdowns (defined as “T1”) and 2. Following the end of the first wave of COVID-19 lockdowns (defined as “T2”). The results indicated that nomophobia, social affiliation, and sleep hours affect PSU. In addition, the indirect effect of the number of sleep hours on stress and PSU was found to be significant in T2 and in T1. The findings and their implications are discussed.