It was not that long ago where people were banning plants from bedrooms because they mistakenly believed that plants "steal" oxygen at night. Now we seem to have come full circle with allegations that plants produce oxygen at night. In general, during photosynthesis (in the presence of sunlight) plants do use CO2 and emit oxygen, both in rather small quantities. When the sun goes down, photosynthesis stops and the use of CO2 and production of oxygen pretty much stops. There may be a slight tipping toward CO2 production at night, but it is very small and over the course of any 24 hour period, plants produce more oxygen than CO2. In balance, then, plants are beneficial in that regard.
That said, Gene is correct in writing that the overall effect is rather marginal unless you are living in a room or house that is crammed full of plants. This also applies to the NASA studies regarding the air-cleaning ability of plants; they are greatly exaggerated.
Select plants based on the available light and enjoy plants because they look nice and make you happy, not because of some exaggerated health benefits.
At least my plants seem to be 
