Because plant on respiration gives co2, because the absence
of light. If light is present the released co2 is again
absorbed in the process of photosynthesis. So in nights if
we sleep under big trees we do not have sufficient amount of
oxygen.

cause plants on respiration gives co2, because the absence
of light. If light is present the released co2 is again
absorbed in the process of photosynthesis. So in nights if
we sleep under big trees we do not have sufficient amount of
oxygen.
 3th standard science

it is said so because during night time plants also take up 
oxygen and give out carbondioxide .

if we sleep under the trees so it can harm are body because at night they leave 
carbon dioxide which is harmful for our body.