In reply to Cthulhu:
> (In reply to LastBoyScout)
>
> I also vaguely remember this from school, but we have a permanently open vent on the window, so not too worried about it...
Depends on the type of photosynthesis. Some plants fix (or at least absorb) CO2 at night to avoid having to open stomata during the day when water loss (through transpiration) would be too high. Cacti or orchids, for example, use this approach. Bromeliads, too.
In reality, the difference it will make to the oxygen/ co2 balance in the room is next to nothing, even if you're sleeping in a greenhouse!