A night guard is a hard plastic retainer placed over either the top or bottom row of your teeth. It is designed to help prevent or slow the damage done to teeth by grinding or clenching. Nighttime grinding and clenching can wear down the tooth enamel, eventually leading to major dental procedures such as root canals, tooth extractions, crown placements and oral surgery. Night guards can also help sufferers of disorders of the temporomandibular joint (TMJ). With a properly designed night guard, the wear on this enamel is decreased, therefore slowing the damage to the teeth.