Actually, plants DO give off carbon dioxide at night because there is no sunlight for them to be converting CO2 to food and oxygen. Just like us, plants release CO2 as part of their living processes, but during the day they produce much more 02. It is for this reason that plants are removed from the room of a sick person at night to keep the air fresher.
Plants absorb oxygen from the atmosphere around them and release carbon dioxide during day time. But, they do opposite at night. Therefore, some people says "do not sleep under tree at night it is not good for our ventilation".