Not being able to focus for 2-3 hours.
Not training your senses to do that.
Eating more than you body needs, leading to lethargy, leading to lack of focus
Not choosing one thing and giving it all your time, attention and energy during those focus hours
Equating sitting in front of the computer with productivity,
Thinking it is not possible for you to focus for 2 hours
And then not focusing even for 1 hour.
What would you add to this list?