Why is it good to not sit under a tree at night? Can you relate it with photosynthesis and respiration?

Dear student,

Since plants perform photosynthesis only during daytime in presence of light, thus at night there is no photosynthesis.
Thus plants do not release oxygen at night. 

They only perform respiration that time and as a result give out carbon dioxide. It is therefore not advisable to sit under a tree at night due as excess of carbon dioxide can be harmful.

Regards