The wish and aim of every student reading for any exam is to read for long hours without loosing focus. But unfortunately, most times you start to feeling dizzy.
Here are some reasons why you feel dizzy and the cure not to sleep once you open your books for reading.
1. Reading After Eating.
After eating your food, it is good you relax for some minutes before going to study, give your body little rest before you start work. Eat small food at night, drink water while eating, avoid you that can cause dizziness such as alcohol, caffeine, etc.
2. Lack Of Rest.
Get enough sleep when you are back from school, sleeping in the afternoon after lectures will help you read longer at night. Even when you don’t have the chance to take afternoon sleep, ensure you don’t sit on a comfortable chair while reading.
3. Too Conducive Environment.
Its good to study in a cool environment but making it too comfort for yourSelf is the problem. Eg, Reading on top of your bed, sitting on a warm chair while reading, reading inside air condition.
Read more
Source https://www.longevity.com.ng/2019/03/08/the-causes-and-remedies-why-you-fall-asleep-immediately-you-start-reading-a-book/

Very true...most esp reading in air con...took a while before I realise it was my achilles heel

Yes bro, too cool atmosphere is not good

The points are good however point three is not totally correct. There are persons that read on ac yet they don't fall asleep while others do and they sleep. This illustration is enough to invalidate point three. Personally I don't read were there is ac or fan but the environment has to be cool whereas my colleagues do yet they don't sleep. Infact if there is no ac or fan they can't study. They feel uncomfortable or can't assimilate well. Secondly you mentioned reading position such as reading on top of your bed, sitting on a warm chair while reading, these points are totally correct. Although some persons may feel uncomfortable yet others would gladly use.
Lack of interest is a major cause to sleeping when reading. Haven't you wonder why many can easily apply those points you raised yet they wouldn't sleep? For example a student can easily watch movie on his bed but may find it difficult to study.

Asuustrike, what work for you may not work for another person