The visual apparatus is very dominant in human beings
It engages us in external worlds. When you close your eyes you can still hear , smell and feel, but half of the world is shut off. When your half of the world is off your internalization begins then and happens the best when your eyes are closed. When you’re doing asanas, you want to internalize everything what is happening inside you . so, for the majority of yogasanas, the eyes should be closed.
How should we breathe during asanas?
Never breathe through your mouth .
There are benefits of breathing through the nose: purification of the air, air temperature is adjusted to your body temperature before it enters the lungs.
When doing the asanas , to bring the body to ease, breathe slightly deeper than normal. Overtime, in any asana, your breathe should become normal. This is not an aerobic exercise- asanas are about building internal strength of the whole system. Within a few weeks of practice, your system will become more capable and your pulse rate will decrease.