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I am doing my PPL course in the Czech Republic and here they have the following question in tests: when during a day is the resistance of a human organism to hypoxia the best? There are 3 possible answers:

  • 3 am
  • 12 am
  • 5 pm

What is the correct answer and why? Is a human's ability to resist hypoxia connected somehow with the time of day? FAA book doesn't have a word about it.

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    $\begingroup$ If it's on the test, it stands to reason that it's in the study material you're expected to use, because that study material would be adapted to the test you'll be taking. What do your Czech books say on the matter? $\endgroup$ – a CVn Feb 11 at 8:52
  • $\begingroup$ @CVn couldn't find anything for now. I prefer the FAA book because it's generally much more detailed and better written than the local books. $\endgroup$ – Kirill Metrik Feb 11 at 17:06
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    $\begingroup$ Hmmm. In my experience, tests aren't designed to facts in general. They are designed to a specific curriculum, and that will most closely match the books they assigned. For instance, your course may make certain assumptions, e.g. European VFR operation, which the test questions will also assume. If you create a more general answer, it may not match the course. Thus, you can get burned by a "bad" test. $\endgroup$ – Harper - Reinstate Monica Feb 11 at 22:59
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    $\begingroup$ Mildly reduced oxygen saturation can impair night vision without causing other noticeable symptoms. The FAA PHAK mentions this in chapter 17. Maybe this is what the test author was trying to get at with their poorly-worded question. $\endgroup$ – pericynthion Feb 12 at 0:25
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    $\begingroup$ I have not found a reference to answer this question other than a study of hypoxia resistance in rats at different times of the day. In Russian. $\endgroup$ – Juan Jimenez Feb 13 at 15:00
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As noted by others here, PPL course questions might not be designed according to pure deductive or inductive reasoning. My approach would have been the following:

Medical-related aviation questions tend to base their background on "old" studies, so let's have a look at Van Liere, E. J. (1964). Resistance to Hypoxia. Archives of Internal Medicine, 113(3), 418.:

As a corollary, it would be anticipated that if the metabolic rate of an animal were elevated, the ability to withstand hypoxia would be decreased. Zarrowetal in 1951 demonstrated that hyperthyroid states decrease tolerance to hypoxia. The thyroid-stimulating hormone (TSH), thyroxin, and dinitrophenol all decrease hypoxic resistance.

TSH has a circadian rhythm (24 hour cycle) and levels peak between midnight and 6am (refer to https://medicalsciences.stackexchange.com/a/4034/15791). Therefore, 3am and 12am are not an option, which leaves us with 5pm.

I'm aware that you stated that the correct answer is 3am, and I'll definitely check back with my medical colleagues on that, but the question in itself is clearly not well-defined within the visible context on this platform.

As others mentioned, one measure of gauging the depression of cellular oxidation is metabolic rate, for which in 2014 according to Examining Variations of Resting Metabolic Rate of Adults: A Public Health Perspective no single value for RMR is appropriate for all adults.

Last but not least, if we talk about hypobaric hypoxia, conclusions change again.

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It depends upon what you mean by hypoxia resistance. Keep in mind you may have to take into account hypoxia’s effects on various organs and organ systems of the body and the effects their impairment at different hours. I’m not going to give away an answer for a test, but that is an additional clue for you to go on. Good luck.

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  • $\begingroup$ I've checked with the answers and the correct answer is 3am. Still have no idea why... $\endgroup$ – Kirill Metrik Feb 17 at 21:27
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Half of the challenge behind piloting exams is understanding what the question is asking. I think this is one example of many poorly-worded questions. AFAIK hypoxia doesn't care what time of day it is, it's all about absorbing oxygen. However, some of the symptoms of hypoxia include reduced colour and night vision, which is bad news for night flight.

If the question doesn't make sense, look at the answers for clues. Out of the 3 options here, 2 of them are very similar - times where it is nighttime year round, and one where it is daylight (varying with latitude and season of course).

So the exam is asking you to choose between day and night, essentially. The symptoms of hypoxia are more abundant at night, so I think you have your answer there.

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  • $\begingroup$ Psst... above and below certain latitudes, midnight and 3am can be daylight during certain times of the year. ;) $\endgroup$ – FreeMan Mar 6 at 14:41

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