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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?

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    $\begingroup$ I have to wonder about this, since in most of the places I've lived during my adult life, I'd have to dig a hole - often a pretty deep one - to get to 5,000 ft. $\endgroup$ – jamesqf Jan 12 '17 at 5:31
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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 & night both. Air Force was, IIRC, the same 10k, with no difference in the rules day vs night.

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  • $\begingroup$ Primarily because of this physiological effect, I use supplemental oxygen at night above 5,000', regardless of the regulations. $\endgroup$ – ammPilot Jan 12 '17 at 6:58
  • $\begingroup$ And likely it's because we don't use the same eye function at night, the night vision is primarily based on rod cells. Rods are already working at an incredible sensitivity, lack of oxygen quickly destroys this sensitivity. $\endgroup$ – mins Jan 12 '17 at 7:18
  • $\begingroup$ The question was edited by @pondlife and unintentionally changed I believe. I never meant to "ask" if FARs have different regulations for day and night. I was stating the fact that no such regulations exist. I've re-edited the question now to make this clear. $\endgroup$ – RaajTram Jan 12 '17 at 22:20
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Humans are diurnal. At night, we get sleepy. The effect is worse when we're extremely tired.

As a result, we are more susceptible to things that affect cognitive processing during those times. Oxygen inadequacy or deprivation (meaning a reduction below the level we are physiologically accustomed to) is one of those things.

It's nothing to do with the actual amount of oxygen at a given altitude changing at night, just our ability to ignore the effect of the existing changes compared to ground level when it is night time.

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The amount of oxygen in the air is the same day or night. The big difference - which isn't really explained in the part of the PHAK you quoted - is that at night your eyes work differently.

Your eyes have two different types of photoreceptors: rods and cones. Cones work best with high light levels and rods work best at low light levels. So during the day, when there's plenty of light, you're seeing primarily with the cones. At night, it's the opposite: your vision depends mostly on the rods.

But, rods are very sensitive to oxygen levels. As the level decreases, they quickly stop working effectively. During the day this doesn't matter because you're not using them anyway, but at night it does matter and you can lose night vision at relatively low altitudes. This is why the PHAK recommends supplemental oxygen at night: it's to make sure that the rods continue to work effectively.

The PHAK has a lot of information about "vision in flight", starting on page 17-19. Page 17-24 explains the need for oxygen at night:

Unaided night vision depends on optimum function and sensitivity of the rods of the retina. Lack of oxygen to the rods (hypoxia) significantly reduces their sensitivity. Sharp clear vision (with the best being equal to 20–20 vision) requires significant oxygen especially at night. Without supplemental oxygen, an individual’s night vision declines measurably at pressure altitudes above 4,000 feet. As altitude increases, the available oxygen decreases, degrading night vision. Compounding the problem is fatigue, which minimizes physiological well being. Adding fatigue to high altitude exposure is a recipe for disaster. In fact, if flying at night at an altitude of 12,000 feet, the pilot may actually see elements of his or her normal vision missing or not in focus

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  • $\begingroup$ Somehow I suspect the PHAK was written by/for sedentary people living near sea level, rather than the (rather large) fraction of the population who've lived most of their lives above 4,000 ft. Not to mention those of us who enjoy spending days hiking up mountains 12K ft or more :-) $\endgroup$ – jamesqf Jan 12 '17 at 20:27

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