Human beings and plants absorb and expel different gasses from their systems as needed. In human beings, oxygen is important for the functioning of the cells. Oxygen is the fuel our body cells use to convert the food we eat into energy. During the energy generation process, carbon dioxide is produced as waste, and the body gets rid of it through respiration. Thus, during respiration, human beings take in oxygen and remove carbon dioxide based on the body's requirements.
During photosynthesis, which occurs during the day, plants use light energy, carbon dioxide, and water to produce food/ energy. Oxygen is also produced and released as waste. In the dark, plants take in oxygen for respiration and expel carbon dioxide.
At night, both plants and human beings use up oxygen, as it is available for both organisms. Plants and human beings do not utilize the net available atmospheric oxygen. In addition, the current number of organisms is incapable of using all the atmospheric oxygen available. Competition for oxygen between the two would only occur in an air-tight or poorly ventilated room with inadequate or no access to atmospheric air.