I want to study 8-9 hrs per day. But ending up 6-7 hrs. How to increase those 1-2 hr more hours per day.

Im currently unemployed and preparing for a job. And I'm full time free. So I think my time is being under-utilised. After studying for 6-7 hrs , im feeling exhausted and tired. So please advice me something.

Note :- I use Forest for tracking my studying hours.