Sinus bradycardia means that the heart is less than 60 bpm. Whether it can cause any harm or not depends on how low the heart rate is and if it is due to some disease process and is causing any symptoms. It should first be thoroughly investigated so that we can find a cause for it. An echo, 24 hr holter should be done and you should consult a cardiologist
. In some people and especially atheletes who exercise a lot normal resting heart rate is low and it does not cause any harm and nothing much needs to be done, but that can be decided only after evaluation, so please consult a cardiologist.