A heart rate of 90 beats/minute while sleeping is most likely caused by a pathologic disease, which may be dangerous if the primary disease is severe, but it is also possible that the primary disease is not dangerous and the patient will just have noticeable symptoms. 1. A heart rate of 90 beats/minute during sleep may suffer from acute myocardial infarction attack, acute heart failure attack and so on. But it may also be caused by other non-critical pathologic diseases, such as electrolyte metabolism disorders, angina attacks, respiratory diseases, hypoglycemic episodes and so on. 2. Most people’s heart rate is on the low side when they sleep at night due to vagal excitation, usually at 60-70 beats/min, and most people may have symptoms of panic if the heart rate reaches 90 beats/min. 3. If the sudden onset of acute myocardial infarction at night or due to the lying position leads to increased return blood flow, causing acute heart failure, resulting in a fast heart rate of 90 beats / min or even higher, this situation is generally more critical, need to go to the hospital as soon as possible to emergency treatment, if not treated in time, there may be a life-threatening situation. 4. Arrhythmic attacks at night, unstable angina attacks or the presence of respiratory diseases lying down when dyspnea can also cause a rapid heart rate, nighttime hypoglycemic reaction can also lead to a rapid heart rate. The symptoms can be improved by adjusting the position or taking emergency medication, but if the symptoms continue to be unrelieved, it is important to go to the hospital as soon as possible. If your heart rate suddenly increases to 90 beats per minute while you are sleeping and you experience uncomfortable symptoms, go to the hospital as soon as possible.