A heart rate of less than 50 beats per minute during sleep is normal in most cases, but a heart rate of less than 40 beats per minute, a feeling of suffocation, and dyspnea are not normal.
A heart rate of less than 60 beats/minute at rest is sinus bradycardia. But in sleep heart rate can be reduced to less than 50 times / min, belongs to the physiological sinus bradycardia, is normal, such as no other symptoms are generally not dealt with as a disease, generally will not produce harm.
If the heart rate is lower than 40 beats/minute during sleep, or there is a sense of suffocation, dyspnea, panic, palpitations, arrhythmia, etc. at 40-50 beats/minute, it is generally considered to be a pathologic bradycardia, which may be caused by high vagal tone, and can be treated in the hospital with oxygen inhalation, atropine, and other treatments.