In higher altitudes, the air is usually colder, less dense, and has less oxygen molecules. This means that if you were there, you would need to take more breaths to get the same amount of oxygen as you would at lower altitudes. Higher elevation makes it even harder to breathe.
The length of daylight and nighttime are of equal length on an equinox because the center of the sun spends a roughly equal amount of time above and below the horizon at every location on the Earth.
