Never play any tourney lasting more than three hours.
I remember a few years ago staring at a monitor for a few hours and suddenly being unable to see anything on screen. Scary at the time.
I stick to non-turbo tourneys with 30 or less players and always over within 3 hours.
I have had that happen to me once about 25 years ago, taking upgrading for my GED, this was the first time I had spent an extended time in front of the monitor, all of a sudden I could not see anything directly in front of me, only through peripheral vision was I able to see.
I understand the fear, but this was only because my eyes were not use to the computer monitor as I didn't have a pc of my own at the time.
Have not had an episode since.
If this is a reoccurring thing for you, seek an eye doctor.
drink beer
As much as I like to drink while playing poker, I would advise against this as alcohol makes you loose.
As for complete focus while playing a tournament, I lack the focus in the early stages and rely upon this forum and random internet searches to cut the boredom and play only solid ABC poker until close to or in the money, then all my focus goes to the table.
You'll be surprised how much focus you have when you are in the money.
"What about the reads you miss on opponents?" you might ask.
I use pokertrackers HUD feature to keep an eye on the reads in the early stages.