A Nightguard is an appliance fabricated from acrylic to cover the biting surfaces of the teeth. Although nightguards can be soft or hard, and cover either the upper or lower teeth, their function is to prevent the biting surfaces of the teeth from coming into contact and thereby preventing grinding or breaking of dental restorations and teeth. Nightguards are prescribed to prevent further destruction of teeth for people who are grinding their teeth and are also used in many cases after a large number of crowns or other restorations have been placed on the teeth to prevent the destruction of newly fabricated restorations. This picture shows a hard acrylic nightguard covering the biting surfaces of the upper teeth. Although a nightguard is usually worn during sleeping hours, your dentist may also prescribe it to be worn during the day, depending on your specific situation.