Patient-friendly definition from the DentistsDirectory.org dental glossary.
A nightguard — also called an occlusal splint, bite guard, or bruxism appliance — is a custom-fabricated removable device worn over the teeth during sleep to protect them from the destructive forces of grinding and clenching (bruxism). It is made from hard or semi-hard acrylic, precisely fitted to the patient's teeth from impressions or a digital scan, and typically covers either the upper or lower arch.
A nightguard does not stop grinding — the neurological reflex that drives bruxism continues. What it does is eliminate direct tooth-to-tooth contact during grinding, so the appliance absorbs the wear instead of the teeth. Over time, a nightguard shows visible wear on its surface that would otherwise have occurred on the patient's enamel. By preventing this wear, it also reduces the muscle fatigue and jaw soreness that heavy clenchers often wake up with, and protects crowns, veneers, and other restorations from fracture.
Over-the-counter boil-and-bite mouth guards are available at pharmacies, but dental professionals generally recommend custom-fabricated nightguards for long-term bruxism management. Custom appliances fit precisely, stay in place better during sleep, do not interfere with breathing, and are fabricated in the correct thickness to absorb forces without destabilising the bite. OTC guards can be helpful as a temporary measure or for patients who frequently lose or damage their custom guards. A nightguard should be cleaned after each use and brought to dental appointments for assessment of wear.