This is true.Is it because you're not really thinking about what exactly it is you're memorising? Try to understand the content. Many other factors can contribute to your problem. Another may be because you're not taking enough breaks. When you try to study for long periods of time, your capacity to take in information and focus decreases exponentially.
It never hurts to understand the intricacies of stuff.^^^ not gonna work for subjects like history where u just need to remember stuff