This depends on the subject and material you are studying. For example a lot of students find it harder to study math and science while others find it harder to take free response tests versus multiple choice tests. Generally speaking the more material you need to study the harder it will be. But the best way to combat this issue is to spread your studying into small chunks that are manageable. So if you are going to study for chemistry, before you start make a clear goal about what you want to know before the end of your study session and get all your materials ready. Now that you did that turn on a timer for 25 minutes to review a single topic, then take a 5 minute break, then do another timer and do this again and again until you feel confident in the material you study. managing your time is the key to studying and this method can be applied to any subject in school.