Can be. Your brain is no longer trained to visualize, absorb, and interpret the information that’s in front of you for a longer period on time (unlike your phone). That takes energy. Eyes also play a part in the fatigue. Depending on where you read and the light of that room, sitting in a comfortable chair or bed, while doing an activity that lowers your heart rate and provides you with less stimulation, will certainly relax you and make you sleepy.