Why do we need to fly at lower altitudes at night to prevent hypoxia? FAA recommends general aviation aircraft to stay below 5,000 ft at night time.
I thought the reason is that oxygen usage with our eye's rods since the rods uses much oxygen and rods can get hypoxia which makes tunnel vision and eventually not able to see outside.
But one of my friends claimed the reason why we are supposed to fly at a lower altitude to prevent from hypoxia is that at night there is higher pressure near the ground which makes fewer air molecules at high altitude at night than daytime.


Is this true that we get hypoxia more easily at night time because of pressure and that's the reason why FAA recommends us to fly below 5000ft?
If the question number 1 is not true (from my understanding), is rods problem the only reason why FAA recommeds us to fly below 5000ft at night time?
The main reason stated by aviation authorities is due to the impairment of vision, your rods provide the majority of your sight in low light conditions and are affected by the lower oxygen levels. However, your colour perception (provided by your eye's cones) is also affected and this is already degraded at night due to the lower light levels.

The below quote is from an EASA General Aviation Safety Briefing on Preventing Hypoxia


  Hypoxia also impairs night vision. Because the rod cells in the eye, which give us night vision, require a lot of oxygen, a lack of oxygen causes visual impairment.


Regarding your friend's point on the diurnal atmospheric changes, this should not be a factor as the suggested altitudes by the aviation authorities on hypoxia prevention should be given as a pressure altitude (i.e. the altitude with the standard altimeter setting of 29.92 inHg or 1013Hpa)

Hope this helps