Falling asleep while reading is most likely the result of being in a relaxed state. If an individual frequently falls asleep during activities other than reading, a sleep disorder may be a possible cause.
If sleep disorders, including narcolepsy, hypersomnia and sleep apnea, are ruled out, a person could fall asleep while reading due to a number of reasons. Most people tend to slow their breathing while he or she are reading and he or she are likely to be sitting in a comfortable position. When someone is reading, his or her mind is often free from distracting thoughts as he or she focuses on the reading material. All of these conditions could contribute to being in a relaxed state.
Aside from being relaxed and comfortable, there could be other reasons for falling asleep while reading. For example, if a person is reading at night, he or she could simply be tired of fatigued and his or her brain may just be ready for sleep. If the person considers the reading material boring, it may keep him or her from being alert and focused, making it easier to fall asleep.