Does oxygen become less available during night-time? The following excerpt is from the FAA's Pilot's Handbook of Aeronautical Knowledge (page 7-37):


  Some people flying above 10,000 feet during the day may experience
  disorientation due to the lack of adequate oxygen. At night,
  especially when fatigued, these effects may occur as low as 5,000
  feet. Therefore, for optimum protection, pilots are encouraged to use
  supplemental oxygen above 10,000 feet cabin altitude during the day
  and above 5,000 feet at night.


The book, in vague terms, suggests that the amount of oxygen (i.e. oxygen molecules) available at a certain altitude at night is less than in daytime. 

The regulations do not differentiate between day and night in regards to supplemental oxygen.

What would be the scientific reasoning behind this?
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.