How does regular exercise to reduce the effects of mental stress

They help us reduce mental stress because during training and regular physical activity we become very focused and can achieve a mindful state as it is often called in psychology. Mindful states have been proven to effectively reduce our stress levels and have a positive impact on our mental hygiene. There is even a type of therapy based on this called mindfulness-based-stress-reduction therapy which battles this problem. 

The apple is red lol