Why should you should not sit under tree during night?

Well no.

General knowledge is that in the morning trees take in sunlight to photosynthesis, to do so they take in CO2 and release oxygen. however at night, without the presence of the sun, they do the opposite. and if you are sleeping under a tree at night, you are breathing in more CO2 than oxygen, which affects your health pretty bad. Brain gets dopey, body feels weak etc etc

And plus, who knows what will drop on your head too. A branch?