The place where respiration takes place is mitochondria, which is expressed as: organic matter+oxygen → carbon dioxide+water+energy;

The respiration function of plants is to consume oxygen and release carbon dioxide. At night, plants do not carry out photosynthesis, but they have a strong respiratory function, absorbing a lot of oxygen and releasing a lot of carbon dioxide. So in the morning, the oxygen concentration in the forest is very small, and the carbon dioxide concentration is very high, which is not conducive to people's breathing, so don't choose to go to the forest for exercise in the morning. On a sunny afternoon, the plants had a day of photosynthesis and gathered a lot of oxygen, so they went to the forest at this time.

So the answer is: at night, plants don't carry out photosynthesis, but they breathe strongly, absorbing a lot of oxygen and releasing a lot of carbon dioxide. So the concentration of carbon dioxide in the forest is very high in the morning, which is not conducive to people's breathing, so don't choose to go to the forest for exercise in the morning. On a sunny afternoon, plants carry out photosynthesis for a day and gather a lot of oxygen. At this time, walking in the forest is beneficial to human health.