I always forget everything i studied for hours. what can be the cause of this ?

Memory retention: The answer is in your sleep and study patterns. Studies have shown that regular rest after study can increase retention. Studying all night without sleep can cause short term memory loss. Different study methods seem to help retention in different persons. A combination of writing, listening to and reading the material can help retention. Proper hydration (3.5 liters in men) and nutrition can help

Exam!: There are many possible causes to your problem of forgetting what you study including anxiety, depression, lack of sleep, drugs, alcohol, poor nutrition, food & other allergies, poor physical conditioning, & illnesses such as thyroid conditions, diabetes & infectious diseases. I therefore encourage you to get a complete check-up from a Family Doctor as soon as possible. I wish you well!