1 Eyes get tired of movement
Reading requires constant eye movement. This movement could let the eye tire and so you might feel like closing them. This can make you feel sleepy.
2 The brain gets exhausted
The reading process requires the brain to keep processing texts and converting them to meaningful words. This process can tire the brain thus lead to the state of sleepiness.
3 Your position is comfortable
Most people sit comfortably while reading. The fact that most muscles are allowed to relax in a comfortable position can make the person who reads feel sleepy.
4 The book is not interesting
If the book you are reading is not interesting enough then your mind will wander and this could cause sleepiness. (See Why some people find it hard to read a book?)
5 You read in bed
When a person gets into bed his subconscious mind automatically recalls the sleeping process. When you read in your bed you will actually be asking your mind to make you sleepy.
6 Too much information
If you are reading fast or receiving too much information then your brain might get overloaded and you might feel sleepy. (See Why is it so hard to read on a computer screen?)
7 You don't read often
Reading requires a certain amount of effort to be done by the brain and the eyes. If you don't read often then probably you will find the reading process tiring and you will feel like falling asleep.
8 Bad lighting
If the lighting is poor then your eyes will need to do more effort. This will make you more likely to feel sleepy.