It can be caused by a variety of factors such as disease, medication, environment and personal, as follows.
Disease factors: such as postural hypotension, carotid sinus syndrome, pathological sinus node syndrome, high atrioventricular block, aortic valve stenosis, etc.
Drug factors: such as sedatives, sleeping pills, antidepressants, narcotics and other drugs used improperly
Environmental factors: being in a hot, crowded environment
Personal factors: such as alcohol consumption, great joy, stress, excitement, overexertion, etc.