1. Sleep at least 7.5 hours a night and make sure your sleep is of good quality.
How do you ensure good quality sleep? Use as little light as possible before sleeping (light in the evening blocks the hormone "melatonin", which you need to sleep well) and make your bedroom as dark as possible. Also, try not to drink caffeine after 2pm and avoid alcohol before going to sleep.
2. Put your phone on do not disturb / airplane mode and work on your most important task of the day for 3-5 hours.
Agree with yourself that the first 3-5 hours of the day you will do nothing but work on the most important task of the day. Don't get distracted by other people, your phone, or unimportant things that come up during the day. Those are for later in the day.
3. Clean up your study or workplace.
Working behind a cleaned up study or workplace ensures that you get more done.
4. Bread or oatmeal in the morning? Get rid of it!
Diet can influence cognitive functions such as concentration and memory. Avoid carbohydrate-rich and processed foods (lots of bread, oatmeal, junk food, etc.) during the first hours of the day. Try this as a test, on day 1 you have a large bowl of oatmeal or a few sandwiches for breakfast. For example, on day 2 you take a bowl of yoghurt with some nuts and fruit in it. You will probably notice the difference immediately when you go to work. In short; save the carbohydrate-rich meals for later in the day. What can you eat then? Salads, eggs, dairy (go for max 5 grams of carbohydrates per 100 grams of dairy), be creative!
5. Take a break from time to time to move
You will see that your brain is a lot fresher after that. Exercise can be as simple as a 5-minute walk. If you study, have a flexible boss or are your own boss, it is also a good idea to do your daily training in the middle of the day.
Get a good night's sleep, start after waking up with the most important task of the day and schedule 3 to 5 hours for it. Put your phone on airplane mode and don't get distracted by other people. Before you start, clean up your workplace and make sure you eat a meal that does not contain very many carbohydrates. After these 3 to 5 hours you have a break, preferably with a walk or a workout. Then you can continue!