What has helped me study better for longer
I want to enumerate some things I've come to learn through my own experiences.
By far, the best time to study is in the morning. This is when you should be most awake. You should be very refreshed and ready to work in the morning. The reason "should" is italicized is because Western culture seems to think that sleep deprivation is normal and okay. It's not. Waking up with an alarm clock is NOT healthy. Your body wakes up naturally when it is done sleeping.
1: Eliminate the alarm clock.
If you have to get up early, go to bed early. Establish a consistent routine. Wake up when you are ready. If you can't wake up early, yet your schedule permits you to wake up late, then wake up late without shame. Sleep as long as necessary.
Now, this next point may be more of a personal thing, but I start to feel sluggish the moment I eat breakfast. No matter what I eat, even if it is just vegetables, I will feel tired after. It must be the process of digestion itself which tires me out. Therefore, I established a diet of intermittent fasting with a split of 16 hours fasting with an 8-hour eating window. So if #2 doesn't apply to you, forget about it. I eat from 12 to 8.
2: Establish an intermittent fasting routine so your first meal of the day is as late as possible.
Now, whenever I try to study, I usually get fatigued after an hour or so to the point where studying is no longer efficient or useful. If I take a break, I will still be tired when I try to go back. I might become even more tired because of the relaxation part. My solution is to take a break, relax, and then go for a 10 minute run.
3: After studying to the point of mental fatigue. Take a short break and then go run for 10 minutes.
After you run for 10 minutes your brain will be more stimulated, less fatigued, and you will be able to get back to work. It may be hard to start running after being tired, but once you start, you will quickly become more energized. Plus, running is good for you. I have had more success with the work/run cycle than the work/rest cycle. It makes sense when you think about it. Muscle fatigue and mental fatigue are two different things, even though the word "fatigue" is used in both cases. When you exercise, you take breaks so your muscles can rest before the next set. However, your brain is different. It needs to rest, but then it needs to be re-stimulated. Exercise seems better at re-stimulating my brain than getting back to work is. I have no scientific proof for this, but it works for me. However, It has been proven that you remember and learn better when your brain is in a more stimulated state.