A nightguard is a mouth guard used overnight to protect your teeth, jawbones and jaw joints from the wear and tear from nighttime grinding and clenching. The official name for these habits is called “bruxism,” and it affects many more people than you would think. Bruxism is considered a sleep movement disorder. A nightguard is a transparent oral appliance that is made to fit your teeth. It may cover all or a portion of your teeth, depending on the type. They can also be fabricated to be worn on the top or bottom.