It's not law that you are on oxygen above 5000 feet at night...merely a recommendation. If you are above 10000 feet for 30 minutes or longer, or above 13,000 feet for any time (here in Canada, at least), day or night, you are required to use supplemental oxygen in an unpressurized aircraft.
Is it a good recommendation? Hard to say...field elevations here can be 3600 feet or more, so its a bit of a hassle for us here...I think it applies to people who live at or near sea level, and thus aren't as acclimatized to higher altitudes. Myself, I judge it on how I feel...at night, I might put the oxy on as low as 8000 feet just because I'm not feeling 100%.
And no, I'm not a smoker.
Time...to un-pimp...ze auto!