The reason why we open our eyes during Zazen.
1) Master Dogen teaches us that the eyes should be open during Zazen in Fukan-zazen-gi and Shobogenzo Zazenigi.
2) In Soto Sect it is the authentic method for us to practice Zazen facing the wall in the case of students.
3) When we close our eyes during Zazen, usually we feel sleepy. At the same time when we close our eyes during Zazen, we are prone to think miscellaneous things in our brain, and so it is much better for us to open eyes during Zazen.
4) During Zazen it is better for us to look at Reality in front of us, and avoid miscellaneous thoughts.
Gudo Wafu Nishijima