[Gideon7]

I found that I could stay down an extra ten or fifteen seconds with my eyes closed than with them open.

That makes sense. The brain uses more oxygen than any other organ in the body, and visual processing uses up a lot of that. By closing your eyes your brain was doing less work.

[Steely Tom]

That makes sense. The brain uses more oxygen than any other organ in the body, and visual processing uses up a lot of that. By closing your eyes your brain was doing less work.

That's what I thought too.

I read somewhere that the neurons that accomplish the processing of visual information make up about 40% of the total mass of the human cerebrum, which tells something of how much value nature puts on vision as a survival function.