Syncope refers to a transient loss of consciousness. The underlying cause is a lack of blood supply to the brain from various causes. There are many causes of inadequate blood supply to the brain, such as cardiac arrest or severe bradycardia, rapid heartbeat (especially ventricular tachycardia), etc. However, approximately two-thirds of clinical syncope belongs to a condition called vasovagal syncope. When it comes to vagus, cardiac autonomic nerves are involved. In the article What is sinus rhythm, we talked about how the heart beats regularly every day and speeds up during stress, exertion, and daytime, and slows down when lying down and resting at night. The main reason for this regular beating of the heart is the regulation of the autonomic nerves. The autonomic nerves that govern the heart and blood vessels actually consist of two components, one is called the sympathetic nerve, which is specifically responsible for making the blood vessels contract and the heart beat faster, and the sympathetic nerve is generally excited when people are nervous, excited and exercising, making the heart beat faster and blood pressure rise, while increasing the blood supply from the heart to the whole body to meet the needs. In contrast, the vagus nerve, once also called the parasympathetic nerve, is responsible for the contraction of blood vessels and the acceleration of the heartbeat. The function of the vagus nerve is the opposite of the sympathetic nerve, it mainly makes the blood vessels dilate and slow down the heart rate, generally mainly when lying down, at night and deep inhalation will stimulate the vagus nerve tension increases. It can be said that stimulating the sympathetic nerve is like stepping on the gas, while stimulating the vagus nerve is like stepping on the brake. These two components are indispensable and mutually constrain each other, which makes our cardiovascular system keep a dynamic balance in coping with sleep, activity, and rest.