Every Second of our life is very precious and teaches us something..!!
Well, it depends how do you want to utilize it? It is good that you at least know that you are not being productive during a certain span of time.
What is my most preferable method to utilize free time “Reading or Solving Puzzles ”, I read Books/Magazines on any random subject which I haven’t studied so far or I solve puzzles online . There may be very people who do not like to read but they have their own ways to utilize free time i.e. travelling, listening music, watching movies etc. However what I suggest is DO what you want to know, what you love and would like to do whenever you are free.
All the best!