Thanks to the 129 people who tried altering their nighttime carbon dioxide levels after my post on this, and who reported back to me. There was no difference between people who pre-registered for the study and people who didn't, on any variable, so I ignored pre-registration.

126 people reported one intervention they performed. The most common was sleeping with a window open:

People generally reported slight but positive changes:

When asked to rate the magnitude of improvement to well-being on a 0 to 5 scale, they averaged 1.4:

I mentioned in the post that succulents could help in theory, but you needed to get the right kind of succulents and you needed at least ten of them. I was skeptical that anyone really got ten succulents in their room, so I wondered whether that might work as a crypto-placebo group.

If so, the intervention failed to separate from placebo. Succulent users had an average improvement of 1.29, compared to about 1.50 for people who did other things. The difference wasn't significant, although admittedly the sample size was low.

Looking at the various groups, the most striking difference was actually people who left a window open (1.57) vs. people who did one of the other named options (1.31). A few people who left windows open mentioned this made their room cooler, which seemed to help with sleep. But this is very post hoc, and this difference wasn't significant either.