When staying up late, many people experience a sudden increase in heart rate for the following two reasons: First, during the time of sleep, it should be the vagus nerve of the human body that holds the dominant position. The vagus nerve can slow down the heart rate and lower the blood pressure, maintaining the effective functioning of the body while allowing various important organs to get sufficient rest. During the day, the sympathetic nerve is dominant, which increases the heart rate and blood pressure, allowing the body to cope with its daily tasks during the day. If you are still working at night during sleep, the vagus and sympathetic nerves will become dysregulated, which will cause a sudden increase in heart rate, resulting in many uncomfortable symptoms. Second, many people choose to drink tea, coffee, smoking during the late night, and these substances can stimulate the vagus nerve, resulting in vagal hyperactivity, causing increased heart rate and blood pressure.