Nightguards are designed for the user from a variety of different durable materials depending on the teeth grinder's needs and are typically between 1 and 3 mm thick.  The condition of your nightguard should call for a night guard based on the severity level which could include a soft night guard, hard night guard, hybrid night guard, or ultra-thin daytime guard.
Approximately, 90% of people elect for an upper nightguard however some people will want a lower nightguard that is better for sleep apnea or if they've had upper mouth dental work.  Dental mouthguards are too thick and it can make it difficult your you to close your lips, lead to increased jaw strain, and can affect energy absorption showing no improved protection between 4 to 6 mm.  When choosing the correct thickness be sure to consult with a dental professional, our dental clinic in London Ontario would be happy to help you with a mouth examination and consult.