It is better to sleep on left side . Sometimes you can sleep on right also. You should not sleep in supine position for a longer time . Reason behind this is , there are big blood vessels in our body which takes blood from our body to heart and from heart purified bleed comes to rest of the body . Whenever you are in supine position this blood vessels get compressed by the uterus . That's why u have to sleep on sides to relieve the compression by growing uterus .Hope you are clear .