[credit_guy]

When I was a kid, my parents held the belief that if you keep lily flowers in a vase in a room with the windows closed overnight, and you sleep in that room, you will get a headache. The belief went on to state there is a small chance of death, for the presumed cause that the flowers somehow consume the oxygen in the room. As an adult, I never revisited this superstition; I don't buy flowers often, but my wife does. For some reason, neither one of us ever buys lilies. It could be that we both hold a remnant of this superstition deep inside our subconscious.

[harpiaharpyja]

Well photosynthesis stops at night but metabolic activity doesn't, so presumably they consume oxygen. No idea how much though.

[whythre]

Plants consume CO2 and ‘exhale’ oxygen. They don’t consume it.

[dimeatree]

Plants do release a bit of CO2 during the night due to respiration, they need sunlight for photosynthesis to occur.