All living beings breathe in order to survive. In the process of breathing they inhale oxygen and exhale carbon dioxide. Oxygen combines with hydrocarbons to produce energy and carbon dioxide.
Plants are living beings and they also breathe to survive. The only difference between animals and plants, in this regard, is that while the animals inhale oxygen and exhale carbon dioxide, the plants inhale carbon dioxide and exhale oxygen during the day and inhale oxygen and exhale carbon dioxide at night.
The leaves of plants take in carbon dioxide from the air during the day. A green substance called chlorophyll, that is present in the leaves, converts it into sugar and water. This process is known as ‘photosynthesis’ and it takes place only in the presence of sunlight. The oxygen produced in this process is discharged into the atmosphere and sugar is used up as food by the plants. This sugar gets converted into carbohydrates, carbonic acid and vitamins.
During the night, when there is no sunlight, the process of photosynthesis comes to an end, during this period plants make use of some part of the sugar produced in the daytime. They take in oxygen from the atmosphere and give out carbon dioxide that is why it is not advisable to sleep under a tree in the night.