According to the passage, what is one problem of using phones too long?

Read the passage and answer the question. Smartphones are useful tools for learning. Students can search for information, watch lessons and practice English with apps. However, using a phone for too long can cause eye strain and poor sleep. Doctors suggest the "20-20-20 rule": every 20 minutes, look at something 20 feet away for 20 seconds. It is also a good idea to stop using screens one hour before bed. According to the passage, what is one problem of using phones too long?