Recently, during an FAA FAAST team webinar, FAA personnel and the CEO of AEROX did a great presentation explaining the history and the science behind when you should use it. For most people, even fit and healthy folks, there is a significant cognitive benefit above 8000' and they were recommending despite what the regs say, if you are flying above 8000' for an hour or more you really should be using supplemental O2. For flight at night, vision is greatly enhanced as well as cognitive ability and the recommendation was 5000' and above at night.
Some one has to set a standard that encompasses all possible levels of fitness, we fit young military aviators in 1967 were required to use oxygen above 5,000 at night versus 10,000 day. Thanks
SWMBO for explaining the reasoning why.