The effect of slumber mist doesn't exactly match a loss of oxygen. It does take 3 rounds (30 seconds) before it starts to take effect, so you could consider the RR to essentially being holding your breath. But falling asleep with no chance of dying isn't quite realistic.
For most games I would say it removes from the air the essential property that sustains both consciousness and fire, and not worry too much about the chemistry. If you want to worry about the chemistry because you have some attachment to modern day scientific thinking in the middle of your magical fantasy, there might be some carbon monoxide mixed in there, maybe some other compounds you really shouldn't be breathing.
In any case, it's described as an effect in the air so it seems reasonable to give a RR bonus for people who were ready (e.g. took a big breath before the spell started). Same could apply to Sudden Light. I wouldn't give them automatic immunity, holding your breath or keeping your eyes closed is somewhat of a challenge if you are in the middle of a melee, but it's easier than if you were totally unprepared.