Does oxygen become less available during night-time?
Night vision is one of the first things affected by reduced oxygen levels, and that effect is masked in daylight, but can become consequential at night.  But no, the O2 molecules are the same night or day.

BTW, airline pilots in our 8000' cabins do not have different guidance for putting on the masks -- it's 10k, day &amp; night both. Air Force was, IIRC, the same 10k, with no difference in the rules day vs night.